How to make 10 post of one category

You can use pre_get_posts and is_category conditional tags to set posts_per_page to 10 for category pages.

Here is a working exacmple

function wpse_posts_cat_pages( $query ) {
    if ( $query->is_category() && $query->is_main_query() ) {
        $query->set( 'posts_per_page', '10' );
    }
}
add_action( 'pre_get_posts', 'wpse_posts_cat_pages' );

What this code do:

• Check if it is the main query $query->is_main_query() and if the page is a category (category.php) page $query->is_category(), and if both of these are valid, the amount of posts are limited to 10, no matter what is set in the back end