It always seemed to me like it was mostly a King's English vs Americanese thing: "forks" in the UK, "fork" in the US.
The American usage is, of course, the correct one.
A "fork" has a "leg," a "branch," or a "tine," depending on what sort of fork it is. Things can have multiple forks, but it's not a fork if it only has one leg, branch, or tine.
Well, maybe if it's a broken fork.
I actually had a flat once caused by a puncture from a one-tined fork laying in the road.
If it'd had two tines, it still would have been one fork, and my tire still would have been forked up.
RichC