I know it depends on many factors, but given a dual core Opteron 170 / 2GB RAM running on cPanel, how many users can you technically fit on this server without any hitches (while still trying to squeeze in as many users as possible)?
Like you said theres alot of factors honestly if aslong as you dont have a few users running HUGE mySQL databases you could put alot. I dont recommend this to ANYONE but a friend of mine had 278 on a single AMD Athlon 2200+ with 1GB of ram. Like i said i wouldnt recommend it. You have a very nice processor which will be excellent for web hosting. Memory wise 2GB is amazing also you could put a very good amount of clients on it depending on your bandwidth/hard drive(s)