You could get away with 5 but you will have mediocre fps and probably not make it into cities. This would probably run 2-3 with decent fps and 2 with good fps. The issue you will run into is memory and the cpu.

As for multiple boxes versus single boxes, I think multiple boxes is slightly more expensive. Single it will need to be beefy to run 5 instances with good fps where as you can lack on performance if buying 5 boxes. Example my box runs 5 clients but has cost me close to $2,200 and if I was going to do hardware boxing I'd grab a couple $500 boxes from dell and a hardware broadcaster (not sure how much these run for).