Source

File: wp-includes/class-wp-site-query.php

private function set_found_sites() {
		global $wpdb;

		if ( $this->query_vars['number'] && ! $this->query_vars['no_found_rows'] ) {
			/**
			 * Filters the query used to retrieve found site count.
			 *
			 * @since 4.6.0
			 *
			 * @param string        $found_sites_query SQL query. Default 'SELECT FOUND_ROWS()'.
			 * @param WP_Site_Query $site_query        The `WP_Site_Query` instance.
			 */
			$found_sites_query = apply_filters( 'found_sites_query', 'SELECT FOUND_ROWS()', $this );

			$this->found_sites = (int) $wpdb->get_var( $found_sites_query );
		}
	}