I'm making my draft tower (horizontal) now and am going with 3" OC. That'll get me 11 faucets in 36" of space. I could go closer (someone once did 2.5") but I think it's enough for me.
That's what I did. I can fit 11 kegs and a fermentation bucket. I have another fridge for fermenting though, but this will fit one more or another 2 kegs. I just wanted to make it easy on building the thing, so 36" is standard length.