Are these private or public servers? It all depends on server quality, slots, speed, etc. If you have a 32 slot public server then people are in an out, and I wouldn't recommend more than 5 servers per box with your settings. If you have very high quality private servers (11 slot 1000 fps) then I would recommend about 10 to keep the quality high!
Nothing is worse than a customer complaining about a "laggy" server and switching companies!
It will depend on load, say you can actually start 15 @ 30 slots each off it, with nobody in those servers, its different from when those servers are full or at half capacity, I say between 7-9 servers @ 20 slots if that, once you get into it, you will find that quality #1 and lower servers more quality.