Change the number of post that are displayed per page for specific CPTs

functions.php

add_action( 'pre_get_posts', 'tht_change_cpt_posts_per_page' );
/**
 * Change Posts Per Page for CPT Archive
 */
function tht_change_cpt_posts_per_page( $query ) {
	
	if( $query->is_main_query() && !is_admin() && (is_post_type_archive( 'definitions' )|| is_post_type_archive( 'question' )) ) {
		$query->set( 'posts_per_page', '18' );
	}
}