Ok, the answer seems to be: at least 200,000 on an Amazon large server.
You have to tweak OF’s caches and you only get acceptable performance when all users are cached - meaning that if the server crashes and many users will try to reconnect, you’re basically screwed. DB must also be up to speed, in my case tweaking postgresql helped boost performance.
Fwiw, the default configuration was only good for 4,000 users, it may be a good idea to offer more sensible defaults out of the box.