Set the number of posts on the page for all site queries in WordPress

set_ posts_per_page.php

// Set the number of posts on the page for all site queries
function tdm_filter_pre_get_posts( $query ) {
    
        $query->set( 'posts_per_page',9 ); // 9 post per page
       
        return $query;
    }
add_filter( 'pre_get_posts', 'tdm_filter_pre_get_posts' );