Personally, I don't think there is a known or documented limit. I have worked with org where were are expecting about 4000 internal and 150,000 community active users which I feel is really low and there are people I know who work with lot more. We have an architect from Salesforce onboard and he hasn't raised any flags regarding users.
The community users have persistent login enabled are supposed to stay logged in for a couple of months.
There are other issues that do stem up from a usability perspective like file storage issues, data storage issues, row locks on update, long running operations etc but if you architect your solution correctly, it should work.