Limit number of checkouts for a specific membership level

my_pmpro_registration_checks.php

<?php

/*
 * Limit checkouts for membership level
 */

function my_pmpro_registration_checks($continue_registration) {

    global $wpdb;

    $member_limit   = 0;             //max number of members for this level
    $level_ids      = array(1,2,3);  //levels to apply this to

    if(in_array($_REQUEST['level'], $level_ids)) {

        //count active memberships for this level
        $count = $wpdb->query("SELECT COUNT(*) FROM $wpdb->pmpro_memberships_users WHERE membership_id=" . $_REQUEST['level'] . " AND status='active'");

        if($count >= $member_limit) {
            $continue_registration = false;
            pmpro_setMessage('The maximum number of members for this level has been reached.', 'pmpro_error');
        }
    }
    return $continue_registration;
}
add_filter('pmpro_registration_checks', 'my_pmpro_registration_checks');