AowVN 250 is the soft limit, but it can exceed this number, as there are no checks from FreeFlarum's side to remove accounts. If it becomes a problem for other forums (e. g.: your forum is too resource intensive), we will ask you to take an action or move your forum elsewhere.
Your user count seems fine, FYI we have forums with even 1,000 accounts and they're fine. That's because inactive accounts aren't a problem, as they're just a data in the database.
We didn't do any stress testing to explore the true limit, as we have no tools to do so. I plan to update this info in the documentation to make it visible to everyone, since this is number 1 asked question and it's only stated in our Terms & Conditions
