WordPress: Category Query - Sets all categories to xx per page

functions.php

add_filter( 'pre_get_posts', 'we_archive_query' );
/**
 * Category Query
 *
 * Sets all categories to 27 per page
 * @since 1.0.0
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 *
 * @param object $query
 */
function we_archive_query( $query ) {
	if( $query->is_main_query() && $query->is_category() ) {
		$query->set( 'posts_per_page', 24 );
	}
}