Depending on how much other stuff is one the circuit
A Nema 15-5 outlet with proper wiring is rated for 15a/120v.
That should be enough for 5 high-end systems with CRTs, assuming you have decent powersupplies. Cheap ones without ActivePFC draw more current.
Based on a %60 efficiency curve and a 350w draw, that would be 3 computers. 350w with CRT is typical for most of the budget systems people around here are buying (winnie 3000, 2x512mb value, 2 hdds, 6600gt)