Paid Memberships Pro: Cap the Number of Members per Level

if ( ! function_exists( 'pmpro_setMessage' ) ) {
    return;
}

// Level ID => maximum active members.
$wpide_pmpro_level_caps = array(
    3 => 100,
);

add_filter( 'pmpro_registration_checks', function ( $continue ) use ( $wpide_pmpro_level_caps ) {
    global $wpdb, $pmpro_level;

    $level_id = isset( $pmpro_level->id ) ? (int) $pmpro_level->id : ( isset( $_REQUEST['level'] ) ? absint( $_REQUEST['level'] ) : 0 );

    if ( ! $continue || empty( $wpide_pmpro_level_caps[ $level_id ] ) || pmpro_hasMembershipLevel( $level_id ) ) {
        return $continue;
    }

    $key   = 'wpide_pmpro_count_' . $level_id;
    $count = get_transient( $key );

    if ( false === $count ) {
        $count = (int) $wpdb->get_var( $wpdb->prepare( "SELECT COUNT(DISTINCT user_id) FROM {$wpdb->pmpro_memberships_users} WHERE membership_id = %d AND status = 'active'", $level_id ) );
        set_transient( $key, $count, 10 * MINUTE_IN_SECONDS );
    }

    if ( $count >= $wpide_pmpro_level_caps[ $level_id ] ) {
        pmpro_setMessage( 'Sorry, this membership is full. Please join the waiting list or choose another level.', 'pmpro_error' );
        return false;
    }

    return $continue;
} );

add_action( 'pmpro_after_change_membership_level', function ( $level_id ) {
    delete_transient( 'wpide_pmpro_count_' . (int) $level_id );
} );