How much Barracks/Gateways does a single CC/Nexus support?

I agree, calculating these kinds of things are tough.

First you have to not calculate what unit producing buildings you need, but rather, what units you're going to build. For example: you can go 5 Rax Reapers, but only 3 Rax if you're doing a mix of Marauders/Marines.

But ok, you've got some build like: 2:1 Marines/Marauders, how many bases do you need to continuously produce that? Man, that'd be annoying to figure out for every combination of units you'd like to build.

Luckly, you don't have to!

Protoss, Terran, Zerg

This will allow you to easily calculate how many bases you need to support the build of X number of type Y units!

I also suggest you try this soft : http://code.google.com/p/scbuildorder/ which allows you to simulate and optimize your build orders for the 3 races.