int $found_posts, WP_Query $query )
Filters the number of found posts for the query.
This filter hook allows developers to adjust the number of posts that WordPress’s WP_Query class reports finding when it runs a query.
This hook is especially useful when developing custom pagination. For instance, if you are declaring a custom offset value in your queries, WordPress will NOT deduct the offset from the the $wp_query->found_posts parameter (for example, if you have 45 usable posts after an offset of 10, WordPress will ignore the offset and still give found_posts a value of 55).
Make sure you haven’t passed no_found_rows in query arguments, Otherwise you will receive a 0 value in return.