/wordpress/plugins/membership/membershipincludes/includes/functions.php

function M_set_level_operations( $levels ) {

	global $M_add_level;

	$user = wp_get_current_user();

	if(count($levels) >= 20) {
		$M_add_level = false;
		$user->remove_cap('M_add_level');
	} else {
		$M_add_level = true;
		$user->add_cap('M_add_level');
	}
}