I've got just a bass and a guitar, and another that's waiting to be sold. I've had more in the past, but I don't see the sense in having 2 of the same instrument. Fretted, fretless, and a 4 if you just have 5's, but I wouldn't buy another fretted 5, it would just get in the way. With guitars you might want a backup if you're gigging a lot, for string breakages and that. It's alright if you're one of these guys with money to burn, and want to have a bunch of boutique basses to sit there and look pretty, but I couldn't part with money like that unless I knew they'd be well used, and would end up paying for themselves, and even then, I'd still prefer to have just one versatile bass, instead of having a whole bunch to cater for different tones etc..