Obviously there is no limitation in software on the number of user accounts. I have a bit limited resources and I was able to test the server with 20,000 user accounts and 20,000 opened connections.
There is obviously limitation in TCP/IP protocol on number of concurrently opened TCP/IP ports. The maximum number is 64k for single IP address. So you can open more ports on the same machine if you setup server to listen on more than one IP address. Assuming of course your operating system allows you to open more ports.
The number of user accounts for single installation depends also on database backend you use. I would not recommend using XMLDB backend for installation with more than 100 users although it was tested for 10,000 registered accounts. If you however use SQL database you should be able to handle millions of accounts in cluster mode installation.