We have a very precise (although one might struggle to describe it) idea of what qualifies as a human leg. The number 2 is basically defined (at least in common usage) as the number of things you have when you have one thing and then another thing. I'd point out that, as you mentioned, this is a different level of abstraction to physics and mathematics.
I would also need to know more QM. I don't know if the theory would actually allow a small fraction of an electron or an electron and a bit to actually exist - common descriptions suggest that it wouldn't. If it doesn't, then electrons could be counted just as much as mathematical objects and legs.
