SrJulius8 the 250 users limit is not enforced, FreeFlarum actually has forums with user counts past this number. We do not support more than 250 users, officially. This means that if it becomes a problem (for our server's resources), we can ask you to migrate away.
It is more problematic with active user count rather than offline users, since these are just a data in the database and do not put stress on the server. Exact numbers are unknown, since we do not have tools to monitor this at the moment.
I am going to word it better in the docs today, to prevent confusion. I should have the time
