The question above was recently posted (with a very thoughtful response) by blog-pal and partial-inspiration-for-the-stuff-you-read-here bikelovejones. I started to respond over at her house, but my answer grew beyond a comment, so I'm taking the inspiration back here where it is sorely needed.
The joke amongst the bikenerderati is that the number of bikes one needs is expressed by the equation n+1, whereby n=the number of bikes currently owned. I totally get that. Poking around the bike shop or the interwebs, I find any number of bikes that, given unlimited funds and a much larger garage, I would want to bring home with me. But when I was a younger man and my BAS (Bicycle Acquisition Syndrome) was at its worst, I was still constrained by the realities of low wages, bills to pay, and limited space. At the most, I think my n reached 3: one road bike, one off-road bike, and one townie/beater. Even today, when I'm lucky enough to have something left in the bank after the bills are paid, those years must have made an impression. I still lust for bikes I don't have, but my "fleet" is pretty tiny by bike geek standards.
Right now, what I consider my "fleet" is at n=1.5: my trusty old Rockhopper and half of a tandem. The Rocky does everything I need a solo bike to do, given my need to get to and from work and my limitations of speed on the road and courage off. The tandem provides quality on-bike time for me and my better half. If I gave up either one, the loss would be felt in a pretty big way. So, in answer to bikelovejones, my "need" threshold is 1.5.
But here's where we get into what a project manager would call a "nice to have": Bikes sometimes fail, even those pieced together from the relatively simple (nay, almost Paleolithic) technology I prefer. When that happens, it would be nice to have a fallback bike rather than feel the pressure to fix the problem like an Indy car pit mechanic. So I'm often tempted to make n=2.5. If nothing else, it would be a luxury to have one bike that could be dedicated to studded tires through the winter months with the "nice" bike wearing regular rubber for the rare day of pleasant weather.
Of course, bikelovejones is right that with additional bikes comes additional hassle. More maintenance. Less space. And when one is saddled (no pun intended) with a touch of obsessive-compulsive disorder, additional bikes means additional things to worry about/obsess over. Frankly, I do enough of that with 1.5 bikes (and even have a bit of OCD left over to obsess over my wife's bike). If I had a massive collection, I'd have to figure in the cost of anti-anxiety medication as a bicycle-related expense.