$post_ids = get_posts( array('fields' => 'ids', 'post_type' => 'class') );
foreach($post_ids as $post_id) {
     $ccc = serialize(get_post_meta( $post_id, $key = 'first_group_class', true ));
     $vvv = substr_count($ccc,"#23#");
}
echo '<br>count= '.$vvv.'<br>';