Create invoices numbers with a system

Found a solution. E.g. add this to your functions.php to get invoicenumbers
NR10001
NR10002
NR10003

function my_pmpro_random_code($code) {
	global $wpdb;
	$result = $wpdb->get_var("SELECT code FROM $wpdb->pmpro_membership_orders ORDER BY id desc LIMIT 1");
	if ($result) {
		$result = substr($result, 2);
		$result = intval($result)+1;
		$code = 'NR'.$result;
	} else {
		$code = 'NR10001';
	}
	return $code;
}
add_filter("pmpro_random_code", "my_pmpro_random_code");