You can do that already with many open source tools, disabling federation from those using activity pub and blocking new sign ups once the user number has reached you desired limit.

The reality is things do not work like that, even to maintain a small community forum you need a small but constant influx of new users as people regularly just disappear/leave as in any social group.

The real challenge is to let decent human people join without the trolls, bots and scammers.