Simple question but difficult to answer. I have one 50mm lens for my one film SLR. It's an old 1.4 "non-Super" Pentax Tak. I answered "2" because I included the 35mm Nikon I own for my DSLR, which is 50mm equivalent on full frame/35mm film. The fixed lens Fuji rangefinder I have has a 45mm focal length, as many/most do, but I didn't include that because it's not "50" though it's in the "normal focal length 50mm range". So, although a simple question, I could have answered 1, 2, or 3... It's tempting to own many - as others here have answered, because they're plentiful and the cheapest/fastest focal length, it's an "essential" focal length, and you may own many cameras systems and have one or more for each... Also, since they're relatively inexpensive on all systems except (goes w/out saying) Leica systems, It's an easy thing to get gas about to buy to check out/fool around with... There are worse ways to waste money than playing around with old 50mm lenses.