How To Force Search API To Reindex a Node / an Entity
You can dive deep into the code of search_api to find solution for issue. See function search_api_entity_update() and search_api_search_api_index_update(). In case reindex number of comment, you can try this code:

// Example reindex after has new comment at node article id 1
$entity = Node::load(1);
$indexes = ContentEntity::getIndexesForEntity($entity);

foreach ($indexes as $index) {
    // Becarefull with indexes. Just for this case.
    // $datasource_id: entity:node (entity type)
    // $updated_item_ids: 1:en (node ID and Language)
    $index-&gt;trackItemsUpdated('entity:node', ['1:en']);
}


You can debug in search_api.module to more info.