How to count items of meta query?

$query->found_posts may be what you are looking for from the code reference:

https://developer.wordpress.org/reference/classes/wp_query/

Note that meta_query expects nested arrays, even if you only have one query. You may need to use:

 $meta_query[0][] = array( .....

You should also take care with the form of the original query. If there are one or more relations defined, you will need to insert your new conditions into the correct relation array.