It's a nice idea to be able to have only one bike. I have a few, but 2 of them are "projects" I'm restoring, & there's a Brompton etc... But 99% of the miles I've ridden have been on the one bike. But I have 3 sets of wheels for it. In winter I have a set of studs on one set of wheels, and this year GP 4 seasons on another, and Marathon's on the final set. Having the spare wheels gives me a higher chance of the bike being ready to ride at any given point. This bike is my race bike, my audax bike, my commuter bike. I'm hoping to have a new bike soon, so I don't have to stress so much about waking up one morning to find that the bike isn't in a ridable condition and I have to take the tram to work...
I've pondered getting another set of wheels, that can take 67mm tyres for off roading, the bike has clearance for it...
How many bikes you need, comes down to your acceptable level of risk that you can't ride on a given day due to a mechanical, or how fast you think you can get a replacement if it was nicked.
J