diff --git a/customfield/field/number/classes/observer.php b/customfield/field/number/classes/observer.php
new file mode 100644
index 00000000000..32438b27222
--- /dev/null
+++ b/customfield/field/number/classes/observer.php
@@ -0,0 +1,112 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace customfield_number;
+
+use customfield_number\local\numberproviders\nofactivities;
+use customfield_number\task\recalculate;
+
+/**
+ * Event observers for customfield_number
+ *
+ * @package    customfield_number
+ * @copyright  Marina Glancy
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class observer {
+
+    /**
+     * When a 'number' custom field was created, schedule recalculation for the field data
+     *
+     * @param \core_customfield\event\field_created $event
+     */
+    public static function field_created(\core_customfield\event\field_created $event): void {
+        $field = $event->get_record_snapshot('customfield_field', $event->objectid);
+        if ($field->type === 'number') {
+            recalculate::schedule_for_field($event->objectid);
+        }
+    }
+
+    /**
+     * When a 'number' custom field was updated, schedule recalculation for the field data
+     *
+     * @param \core_customfield\event\field_updated $event
+     */
+    public static function field_updated(\core_customfield\event\field_updated $event): void {
+        $field = $event->get_record_snapshot('customfield_field', $event->objectid);
+        if ($field->type === 'number') {
+            recalculate::schedule_for_field($event->objectid);
+        }
+    }
+
+    /**
+     * When a course module was created, schedule recalculation for all 'nofactivities' custom fields
+     *
+     * @param \core\event\course_module_created $event
+     */
+    public static function course_module_created(\core\event\course_module_created $event): void {
+        if (self::has_nofactivities_fields()) {
+            recalculate::schedule_for_fieldtype(fieldtype: nofactivities::class,
+                component: 'core_course', area: 'course', instanceid: $event->courseid);
+        }
+    }
+
+    /**
+     * When a course module was deleted, schedule recalculation for all 'nofactivities' custom fields
+     *
+     * @param \core\event\course_module_deleted $event
+     */
+    public static function course_module_deleted(\core\event\course_module_deleted $event): void {
+        if (self::has_nofactivities_fields()) {
+            recalculate::schedule_for_fieldtype(fieldtype: nofactivities::class,
+                component: 'core_course', area: 'course', instanceid: $event->courseid);
+        }
+    }
+
+    /**
+     * When a course module was updated, schedule recalculation for all 'nofactivities' custom fields
+     *
+     * Module visibility may change following an 'updated' event and it will affect the activities count
+     *
+     * @param \core\event\course_module_updated $event
+     */
+    public static function course_module_updated(\core\event\course_module_updated $event): void {
+        if (self::has_nofactivities_fields()) {
+            recalculate::schedule_for_fieldtype(fieldtype: nofactivities::class,
+                component: 'core_course', area: 'course', instanceid: $event->courseid);
+        }
+    }
+
+    /**
+     * Checks if a 'number' field with 'nofactivities' provider exists in the course fields
+     *
+     * This method is very fast, it only performs one DB query and only once per request
+     *
+     * @return bool
+     */
+    protected static function has_nofactivities_fields(): bool {
+        $handler = \core_course\customfield\course_handler::create();
+        foreach ($handler->get_categories_with_fields() as $category) {
+            foreach ($category->get_fields() as $field) {
+                if ($field->get('type') === 'number' &&
+                        $field->get_configdata_property('fieldtype') === nofactivities::class) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+}
diff --git a/customfield/field/number/classes/task/recalculate.php b/customfield/field/number/classes/task/recalculate.php
new file mode 100644
index 00000000000..840447d8f3f
--- /dev/null
+++ b/customfield/field/number/classes/task/recalculate.php
@@ -0,0 +1,118 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace customfield_number\task;
+
+use coding_exception;
+use core\task\adhoc_task;
+use core_customfield\field_controller;
+use customfield_number\provider_base;
+
+/**
+ * Recalculates data for the given number field with a provider
+ *
+ * @since      Moodle 4.5.1
+ * @package    customfield_number
+ * @copyright  Marina Glancy
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class recalculate extends adhoc_task {
+    #[\Override]
+    public function execute() {
+        global $DB;
+        $customdata = $this->get_custom_data();
+        $fieldid = clean_param($customdata->fieldid ?? null, PARAM_INT);
+        $fieldtype = $customdata->fieldtype ?? null;
+
+        // Find all fields that we need to recalculate (either by 'fieldid' or 'fieldtype').
+        $fields = [];
+        if ($fieldid) {
+            try {
+                $fields[] = field_controller::create($fieldid);
+            } catch (\Exception $e) {
+                // Could be a race condition when the field was already deleted by the time ad-hoc task runs.
+                return;
+            }
+        } else if ($fieldtype) {
+            $records = $DB->get_records('customfield_field', ['type' => 'number']);
+            foreach ($records as $record) {
+                $configdata = @json_decode($record->configdata, true);
+                if (($configdata['fieldtype'] ?? '') === $fieldtype) {
+                    $fields[] = field_controller::create(0, $record);
+                }
+            }
+        }
+
+        // Schedule recalculate for each field, checking component, area and the presense of provider.
+        $instanceid = clean_param($customdata->instanceid ?? null, PARAM_INT);
+        foreach ($fields as $field) {
+            if ($this->field_is_scheduled($field) && ($provider = provider_base::instance($field))) {
+                $provider->recalculate($instanceid ?: null);
+            }
+        }
+    }
+
+    /**
+     * Helper method validating that the field should be recalculated
+     *
+     * @param \core_customfield\field_controller $field
+     * @return bool
+     */
+    protected function field_is_scheduled(field_controller $field): bool {
+        $customdata = $this->get_custom_data();
+        if (!empty($customdata->component) && $field->get_handler()->get_component() !== $customdata->component) {
+            return false;
+        }
+        if (!empty($customdata->area) && $field->get_handler()->get_area() !== $customdata->area) {
+            return false;
+        }
+        return true;
+    }
+
+    /**
+     * Schedule recalculation for the given number custom field (and optionally, instanceid)
+     *
+     * @param int $fieldid in of the custom field
+     * @param int|null $instanceid if specified, only recalculates for the given instance id
+     * @return void
+     */
+    public static function schedule_for_field(int $fieldid, ?int $instanceid = null) {
+        $task = new static();
+        $task->set_custom_data(['fieldid' => $fieldid, 'instanceid' => $instanceid]);
+        \core\task\manager::queue_adhoc_task($task, true);
+    }
+
+    /**
+     * Schedule recalculation for all number custom fields that use the provider (optionally with instanceid)
+     *
+     * @param string $fieldtype name of the class extending provider_base
+     * @param string|null $component
+     * @param string|null $area
+     * @param int|null $instanceid
+     * @return void
+     */
+    public static function schedule_for_fieldtype(string $fieldtype, ?string $component = null, ?string $area = null,
+            ?int $instanceid = null) {
+        $task = new static();
+        $task->set_custom_data([
+            'fieldtype' => $fieldtype,
+            'component' => $component,
+            'area' => $area,
+            'instanceid' => $instanceid,
+        ]);
+        \core\task\manager::queue_adhoc_task($task, true);
+    }
+}
diff --git a/customfield/field/number/db/events.php b/customfield/field/number/db/events.php
new file mode 100644
index 00000000000..6ebcb57585e
--- /dev/null
+++ b/customfield/field/number/db/events.php
@@ -0,0 +1,49 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Event observers for Number
+ *
+ * @package    customfield_number
+ * @category   event
+ * @copyright  Marina Glancy
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+$observers = [
+    [
+        'eventname' => core_customfield\event\field_created::class,
+        'callback' => 'customfield_number\observer::field_created',
+    ],
+    [
+        'eventname' => core_customfield\event\field_updated::class,
+        'callback' => 'customfield_number\observer::field_updated',
+    ],
+    [
+        'eventname' => core\event\course_module_created::class,
+        'callback' => 'customfield_number\observer::course_module_created',
+    ],
+    [
+        'eventname' => core\event\course_module_deleted::class,
+        'callback' => 'customfield_number\observer::course_module_deleted',
+    ],
+    [
+        'eventname' => core\event\course_module_updated::class,
+        'callback' => 'customfield_number\observer::course_module_updated',
+    ],
+];
diff --git a/customfield/field/number/tests/observer_test.php b/customfield/field/number/tests/observer_test.php
new file mode 100644
index 00000000000..7dac110bec9
--- /dev/null
+++ b/customfield/field/number/tests/observer_test.php
@@ -0,0 +1,245 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+namespace customfield_number;
+
+use context_module;
+use customfield_number\local\numberproviders\nofactivities;
+use customfield_number\task\recalculate;
+
+/**
+ * Testing event observers
+ *
+ * @covers     \customfield_number\observer
+ * @covers     \customfield_number\task\recalculate
+ * @package    customfield_number
+ * @category   test
+ * @copyright  Marina Glancy
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+final class observer_test extends \advanced_testcase {
+
+    /**
+     * Create a number custom field
+     *
+     * @param array $configdata
+     * @return \customfield_number\field_controller
+     */
+    protected function create_number_custom_field(array $configdata): field_controller {
+        /** @var \core_customfield_generator $generator */
+        $generator = $this->getDataGenerator()->get_plugin_generator('core_customfield');
+
+        // Create a category and field.
+        $category = $generator->create_category();
+        $field = $generator->create_field([
+            'categoryid' => $category->get('id'),
+            'type' => 'number',
+            'configdata' => $configdata,
+        ]);
+        return $field;
+    }
+
+    /**
+     * Helper function that checks if the recalculate ad-hoc task is scheduled
+     *
+     * @param bool $mustbescheduled - when false checks that the adhoc task is NOT scheduled
+     * @return void
+     */
+    protected function ensure_number_adhoc_task_is_scheduled(bool $mustbescheduled): void {
+        $tasks = array_filter(
+            \core\task\manager::get_candidate_adhoc_tasks(time(), 1200, null),
+            fn($task) => $task->classname === '\\' . recalculate::class
+        );
+        if ($mustbescheduled && empty($tasks)) {
+            $this->fail('Recalculate ad-hoc task is not scheduled.');
+        } else if (!$mustbescheduled && !empty($tasks)) {
+            $this->fail('Recalculate ad-hoc task is scheduled when it is not expected.');
+        }
+    }
+
+    /**
+     * Test for observer for field_created event
+     */
+    public function test_field_created(): void {
+        global $DB;
+
+        $this->resetAfterTest();
+        $this->setAdminUser();
+
+        $course1 = $this->getDataGenerator()->create_course();
+        $course2 = $this->getDataGenerator()->create_course();
+        $assigngenerator = $this->getDataGenerator()->get_plugin_generator('mod_assign');
+        $assign1 = $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 1]);
+        $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 1]);
+
+        // Create a number field with a provider.
+        $field = $this->create_number_custom_field(['fieldtype' => nofactivities::class, 'activitytypes' => ['assign', 'forum']]);
+
+        // Execute scheduled ad-hoc tasks and it will populate the data for the course.
+        $this->ensure_number_adhoc_task_is_scheduled(true);
+        $this->run_all_adhoc_tasks();
+
+        $alldata = $DB->get_records_menu('customfield_data',
+            ['fieldid' => $field->get('id')], 'instanceid', 'instanceid, decvalue');
+        $this->assertEquals([$course1->id => 2], $alldata);
+
+        // Creating another field type does not schedule tasks.
+        $this->ensure_number_adhoc_task_is_scheduled(false);
+        $this->getDataGenerator()->get_plugin_generator('core_customfield')->create_field((object)[
+            'categoryid' => $field->get_category()->get('id'),
+            'type' => 'textarea',
+        ]);
+        $this->ensure_number_adhoc_task_is_scheduled(false);
+    }
+
+    /**
+     * Test for observer for field_updated event
+     */
+    public function test_field_updated(): void {
+        global $DB;
+
+        $this->resetAfterTest();
+        $this->setAdminUser();
+
+        $course1 = $this->getDataGenerator()->create_course();
+        $course2 = $this->getDataGenerator()->create_course();
+        /** @var \mod_assign_generator $assigngenerator */
+        $assigngenerator = $this->getDataGenerator()->get_plugin_generator('mod_assign');
+        $assign1 = $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 1]);
+        $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 1]);
+
+        // Create a simple number field.
+        $field = $this->create_number_custom_field([]);
+
+        // There is no data for this field yet.
+        $this->ensure_number_adhoc_task_is_scheduled(true);
+        $this->run_all_adhoc_tasks();
+        $this->assertEmpty($DB->get_records('customfield_data', ['fieldid' => $field->get('id')]));
+
+        // Update this field to use nofactivities as provider.
+        $params = ['fieldtype' => nofactivities::class, 'activitytypes' => ['assign', 'forum']];
+        \core_customfield\api::save_field_configuration($field, (object)['configdata' => $params]);
+
+        // Now an ad-hoc task is scheduled and the data is populated.
+        $this->ensure_number_adhoc_task_is_scheduled(true);
+        $this->run_all_adhoc_tasks();
+
+        $alldata = $DB->get_records_menu(
+            'customfield_data',
+            ['fieldid' => $field->get('id')],
+            'instanceid',
+            'instanceid, decvalue'
+        );
+        $this->assertEquals([$course1->id => 2], $alldata);
+    }
+
+    /**
+     * Test for observer for course_module_created, course_module_updated and course_module_deleted events
+     */
+    public function test_course_module_events(): void {
+        global $DB;
+
+        $this->resetAfterTest();
+        $this->setAdminUser();
+
+        $course1 = $this->getDataGenerator()->create_course();
+        $course2 = $this->getDataGenerator()->create_course();
+
+        // Create a number field with a provider.
+        $field = $this->create_number_custom_field(['fieldtype' => nofactivities::class, 'activitytypes' => ['assign', 'forum']]);
+
+        // There is no data for this field yet.
+        $this->ensure_number_adhoc_task_is_scheduled(true);
+        $this->run_all_adhoc_tasks();
+        $this->assertEmpty($DB->get_records('customfield_data', ['fieldid' => $field->get('id')]));
+
+        // Create modules.
+        $assigngenerator = $this->getDataGenerator()->get_plugin_generator('mod_assign');
+        $assign1 = $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 1]);
+        $assign2 = $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 0]);
+        $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 1]);
+
+        // Execute scheduled ad-hoc tasks and it will populate the data for the course.
+        $this->ensure_number_adhoc_task_is_scheduled(true);
+        $this->run_all_adhoc_tasks();
+
+        $alldata = $DB->get_records_menu('customfield_data',
+            ['fieldid' => $field->get('id')], 'instanceid', 'instanceid, decvalue');
+        $this->assertEquals([$course1->id => 2], $alldata);
+
+        // Update visibility of one module.
+        set_coursemodule_visible($assign2->cmid, 1);
+        [$course, $cm] = get_course_and_cm_from_cmid($assign2->cmid);
+        \core\event\course_module_updated::create_from_cm($cm, context_module::instance($assign2->cmid))->trigger();
+        $this->ensure_number_adhoc_task_is_scheduled(true);
+        $this->run_all_adhoc_tasks();
+
+        $alldata = $DB->get_records_menu('customfield_data',
+            ['fieldid' => $field->get('id')], 'instanceid', 'instanceid, decvalue');
+        $this->assertEquals([$course1->id => 3], $alldata);
+
+        // Delete one module.
+        course_delete_module($assign1->cmid);
+
+        // Execute scheduled ad-hoc tasks and it will update the data for the course.
+        $this->ensure_number_adhoc_task_is_scheduled(true);
+        $this->run_all_adhoc_tasks();
+
+        $alldata = $DB->get_records_menu('customfield_data',
+            ['fieldid' => $field->get('id')], 'instanceid', 'instanceid, decvalue');
+        $this->assertEquals([$course1->id => 2], $alldata);
+    }
+
+    /**
+     * Creating, updating and deleting modules when there are no 'nofactivities' custom fields does not schedule the ad-hoc task
+     *
+     * @return void
+     */
+    public function test_course_module_events_without_custom_fields(): void {
+        global $DB;
+
+        $this->resetAfterTest();
+        $this->setAdminUser();
+
+        $course1 = $this->getDataGenerator()->create_course();
+        $course2 = $this->getDataGenerator()->create_course();
+
+        // Create a number field without a provider.
+        $field = $this->create_number_custom_field([]);
+
+        // Initial ad-hoc task was scheduled.
+        $this->ensure_number_adhoc_task_is_scheduled(true);
+        $this->run_all_adhoc_tasks();
+
+        // Create modules.
+        $assigngenerator = $this->getDataGenerator()->get_plugin_generator('mod_assign');
+        $assign1 = $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 1]);
+        $assign2 = $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 0]);
+        $assigngenerator->create_instance(['course' => $course1->id, 'visible' => 1]);
+
+        $this->ensure_number_adhoc_task_is_scheduled(false);
+
+        // Update visibility of one module.
+        set_coursemodule_visible($assign2->cmid, 1);
+        [$course, $cm] = get_course_and_cm_from_cmid($assign2->cmid);
+        \core\event\course_module_updated::create_from_cm($cm, context_module::instance($assign2->cmid))->trigger();
+        $this->ensure_number_adhoc_task_is_scheduled(false);
+
+        // Delete one module.
+        course_delete_module($assign1->cmid);
+        $this->ensure_number_adhoc_task_is_scheduled(false);
+    }
+}
diff --git a/customfield/field/number/version.php b/customfield/field/number/version.php
index 91c8f4c77b1..6404b38f065 100644
--- a/customfield/field/number/version.php
+++ b/customfield/field/number/version.php
@@ -25,6 +25,6 @@
 defined('MOODLE_INTERNAL') || die;
 
 $plugin->component = 'customfield_number';
-$plugin->version   = 2024100700;
+$plugin->version   = 2024100703;
 $plugin->requires  = 2024100100;
 $plugin->maturity  = MATURITY_STABLE;