I had a 8 core 16GB and running 30 instances without going full processor. But overall, it pretty much depends on which script you plan to use and the core/ram speed. Through my experience, running all bots in 1 server do save you lots of time since you don't have to switch server all the time. But somehow, i would rather choose multiple servers because the less bot you run or push into a server, the more money you will be making.