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// Show 20 posts per page on search, category and archive pages
// Modify condition as needed
/////////////////////////////////////////

function limit_posts_per_search_page() {
	if ( is_search() OR is_category() OR is_archive() )
		set_query_var('posts_per_archive_page', 20); 
}

add_filter('pre_get_posts', 'limit_posts_per_search_page');