If you can own just one bike - but still want to ride anywhere - buy this bike
How many bikes can you realistically own? Apparently the correct number is n+1, as Velominati notes, with an important caveat: "While the minimum number of bikes one should own is three, the correct number is n+1, where n is the number of bikes currently owned. This equation may also be re-written as s-1, where s is the number of bikes owned that would result in separation from your partner."
But if you could really own just one bike, which would it be? I'd want a bike that could do just about everything and not be crazy expensive. It wouldn't have to be perfect at anything, except at doing everything well. I like riding on all terrains, often in the same week, and sometimes on the same ride, so it would need to be an all-seasons commuter, fast enough for weekend group rides, and something I could confidently take off-road when the trails call. It'd also have to be fun and good-looking.
For me, the Raleigh Roker Comp would probably be that bike. I just rode it for a month, and this is what it's all about: