In v8.4 and older, you count the number of UNIX processes.
In v8.5 and newer, arssockd uses a threaded model, and you can get thread info with arssockd -p. I don't believe this has the active number of users though.
It also gets tricky when you're using ODWEK -- the Java API has the option of connection pooling, so you might see one connection for dozens of actual end users.
Fred is right that the System Log is the way to find this out. But number of users in itself isn't terribly useful. What exactly are you trying to monitor?
-JD.