If you want to run a "farm" of several gameservers, consider looking up the concept of "Virtual Private Hosts" as that will help you better manage the user resources. That said the numbers people here will give you are for servers that see a lot of use, and quite frankly many servers people pay for ... don't really see a lot of use
So you also have to account for that.
If you want to run a LOT of gameservers then it can be more economical and easier to control to work out a lease deal with a datacenter that allows you to get servers from them on a "per needed" basis, ie, when you sell one to a customer, than it is to try to build and maintain some uber-computer that can handle 100 gameservers because we're pushing into supercomputer territory there if we're handling new games.
As a rule of thumb I assume 1-6 servers per processor or "core" on the machine, it really depends on a lot of variables. Original Half-Life for example is much less demanding than Left For Dead 2 for example and you could probably push a top-of-the-line computer to that upper limit of six, whilst for the newer games you'll be lucky to get more than one going per core.
The 'problem' if you can call it that is when the server starts, restarts, and loads maps, it will spike the CPU usage of that core, so for newer games that really need the faster processor, this means that every time one server restarts or switches maps, ALL the servers on that core are going to lag.