At http://www.jivesoftware.com/products/openfire/resources/requirements.jsp the section Hardware Recommendations says that it is possible to achieve 10,001 - 25,000 concurrent users on one machine, while having one connection manager on the same machine. I wonder how this might be possible, since XMPP needs one TCP-Connection for every XMPP-Session. As far as I know the number of open TCP-Connections is limited through its Operating System. Do I understand something totally wrong?
The total number of tcp connections will be limited by the OS, so the absolute max is determinate on what OS you run. Unix systems use file descriptors for the socket connections, so it will generally be limited by that number before other factors with large quantities of memory to deal with. Windows may be different, I dont know what sort of limits it has; I think it uses file descriptors too. So even assuming 2^16 file descriptors, thats 65K. So 25K connections is not really unreasonable in terms of what the OS can handle- the bottleneck is almost always with memory or the application itself.
Just to give some more information. In our tests with 64bits OS we got to 250K in a single box. We do not recommend placing too many eggs in the same basket but as you can see you can get very high numbers.