As seen above, the answer is as individual as we are. I always thought 4 minimum for electrics: strat, tele, semi-hollow, LP, but I have purged my LP (actually a Yammy SG) about half a year ago, so for the first time in about 5 years, I am down to 3 electrics, but I now have 3 acoustics parlor, reso, beater. Plus, gotta have a bass, so my minimum is 6 + bass. I rotate almost daily with no case queens whatsoever. I do GAS for a P-90 equipped guitar, but I'm not sure why. That does support JL_LI's theory of n+1 is the answer and n+2 is too much. I kinda like that.