At present I'd say I need three. Although 'need' is certainly a first-world answer.
I split my time living in two entirely different towns for many years. As such, I ended up with two daily drivers. Now that I live in one town full time like a normal person, I still have the two daily driver bikes, and I use them as such (in short if I leave the house I try and do it on the bike, regardless of where I am going or what I need to do).
My original plan was to sell one of the two bikes, but the market is in the tank and I am not willing to almost give away a bike I still put a lot of value in. I have really appreciated having a backup immediately available several times over the last year. Typically that means I have a worn out part I need to replace. Being able to switch lets me put off the repair until when I have the time to do it conveniently.
The third bike I need is the little one. A bike intended to be small (26" wheels) and really light-weight. If I have to take the car into the shop, the bike goes into the back and I ride home, and then ride it back to the shop when its time to pick it up. Its also my vacation bike.
I have a 29er that is also lightweight, but its Size Large frame and wagon wheels make it a major pain to get into the car. Especially with a load of camping gear. So I downsized and its been the perfect solution.
Regardless of any specifics, I'd always answer at least two bikes. You always need a backup if your bike is something more than a toy.