Thread:Extension talk:MediaWikiChat/max concurrent users./reply

It should be able to handle quite a large number depending on how much your server can handle. Most of the processing work of the chat is done client-side with JavaScript, so as long as your server can handle the bandwidth and the MySQL data writing (how messages are handled on the server) you should be good. I can't say for sure that it'll work well with 100+ users since I've never seen that many at once before. The most I've ever witnessed the extension handle is approximately 20 users on Brickimedia, specifically http://en.brickimedia.org.