I know that "index.php?cat=10001" is me, the registered has the following id in the order "index.php?cat=10003","index.php?cat=10004".........
as a result, what will happen when there are more than 99999 members?
cat id will then be 100000, 100001, 100002 etc. It is calculated as 10000 plus member id. There's no limit. Has been asked and answered before. Looking at the code will show you as well.