One or none
The difference between one and none is not always clear.
Explanation
Because we concern us here with mathematics, it sounds reasonable to call one 1 and none 0. That is quite logical. And moreover it becomes abstract. You would think that it should be clear whether the result of a calculation gives 0 or 1.
But that is often not so easy.
Details
You can say: I have two. Or you could say: I have one. And if you don't have anything, you can even say: I don't have one. Of course that's the same as: I have nothing.