Hi pb13 and welcome to the forums!

how many it can support depends on the specs you'd like to achieve.

since you can adjust the maxfps and maxfpsbk, you should be able to get 5 on that box without a worry.
i would suggest maxfps(40+) and maxfpsbk(15+). watch the resolution you are using as that will drive up the # pixels to push

you could just get 5 test accounts and see how they perform.

i'd also suggest getting dual monitors.. it's just cooler :thumbup:

g'luck and have fun