One bike with spare wheels for commuting/touring — sooner or later the mismatch between chain and cassette wear will result in one of the rears becomng unusable long before you'd normally replace the transmission.
Ever heard the phrase "jack of all trades, master of none"?
I've five bikes including a roadie and a 5in full sus, both of which don't see a lot of use but I'd have to be really skint before I'd sell any of them.