It's difficult to say. It really depends on how you are using the software, how many policies, size of packages, are you imaging?
With that many clients, I would have 2-3 com servers setup. CPU and memory typically aren't an issue. Any modern CPU and 16GB of ram should be fine.
I run 3 com servers, 1 internal(16GB), 1 external(4GB), and 1 dedicated for imaging for 1000 clients.