It's a hub server where normal users are not allowed to log in, and no files are shared. It's sole purpose is just to route messages among the other leaf servers.
It's really hard to say what the maximum is. I doubt you'd get that many servers working together given the amount of data that needs to be shared among all the servers. There is no hard limit, but in practice it becomes difficult to use a cluster of that many servers. Mostly we see between 5-7 servers in a cluster.
