Wordpress limits to 30 by default. See the source here:
https://core.trac.wordpress.org/browser/tags/3.8/src/wp-admin/includes/template.php#L556
add_filter( 'postmeta_form_limit' , 'customfield_limit_increase' );
function customfield_limit_increase( $limit ) {
$limit = 60;
return $limit;
}
in your functions.php should set you up.
