You could develop a plugin that checks how many users are online when a user logs in and if a predefined number has been reached prevent the user from signing in.
Yes, I’ve thought about it too. That’s probably a solution for future. But make a linux firewall to deal with the connection amount seems easier. However I’m not sure that it really is. Openfire could keep several connections for one user and use different ports for commands and messages, so the linux firewall will prevent some users to send msgs as the connection max amount on both ports is reached. I mean it also could bee tricky. Something like this.
The client control plugin allows you to limit users based on the client they are using - It does not rate limit, or control the number of users who can access the environment.