Use this snippet:

$user = elgg_get_logged_in_user_entity();

$list = elgg_get_entities_from_metadata(array(
        'type' => 'object',
        'subtype' => 'site_notification',
        'owner_guid' => $user->guid,
        'full_view' => false,
        'metadata_name' => 'read',
        'metadata_value' => false,
        'count' => true,
    ));
    
echo $list;

Thanks again RvR!

Is possible count only unread notifications?

You can try this for unread notifications

    $notifications_no = elgg_get_entities_from_metadata(array(
        'type' => 'object',
        'subtype' => 'site_notification',
        'owner_guid' => elgg_get_logged_in_user_entity(),
        'metadata_name' => 'read',
        'metadata_value' => false,
        'count' => true,
    ));