i run up to 8 bots on core i7 3930k(3940 now available for same money), 32 gb of ram and a geforce 660ti on a 500gb ssd. I run all bots in separated vmware. I guess a setup without vmware could take 10-12
I run 10 clients with I7 and also 16G RAM and I use around the 60% of the ram so I'm sure you could handle 15 clients whitout VMware are any other virtualbox.
but you can only do bgbuddy/GB2/questing/PB and AG because Dungeonbuddy uses way to much CPU that even with 5 clients running I sometimes have a CPU from 80% with 8-cores