There's a number of bot programs listed on SteemTools and they're fairly small and simple programs that I would generally expect to see running on personal computers.

Most bot users only appear to have a handful of accounts, and bots are limited by SteemIt's bandwidth restrictions, which prevents them from spamming nonstop - Usually there's a few minutes between posts. Because of that I think you'd be looking at pretty low requirements in terms of processing power even if someone was running 100 bots.

There is evidence that some users might be running thousands of bots which might require more than one computer. More likely they would run a program that loops through their various accounts, logging in to one, using its voting power/bandwidth, then moving on to the next one and so on.

I think it's unlikely that anyone is running a large server dedicated exclusively to SteemIt bots, although it is possible.