Doing okay so far with just a dual core...albeit a very tweaked system. Oh, and 8GB of system RAM and 768 VRAM. But 60 FPS more or less stable on the main 1920x1200 screen with high textures etc (i.e. Vsync). The slaves are locked to a lower frame rate, but fill nearly that many pixels again between them. Woot?

Yeah, I should probably drop in a quad core eventually. Meh. I want new speakers first, I think.

But if only two cores can do five respectably, eight should be fine for ten - provided you have the memory, bandwidth, etc, and that whatever actual display constellation you want doesn't fry your vid card. But CPU power should not be the limiting factor. 2-3 sessions per core is fairly reasonable if you have appropriate settings and the rest of the system cooperates. Within limits...I don't know that I'd go for 20 even if you had 20 cores somehow. Stability tends to scale a bit worse than the number of instances of the program, so you're looking at over 20x the normal crash risk of a slightly crash-prone client then. Better to break up the boxen at some point below that. Five per machine is okay if it can handle it, ten is extreme but doable...but probably the limit.

Oh, and cost per client is probably somewhat below optimal once we start talking Skulltrail and so forth.