Null is not a number. It's an error.
Null is more than just an error, its more like an absence of substance.
an error is like the square root of negative one, which is also an imaginary number, which is also a number. Or, you could say 1 divided by zero is an error, but its also more or less infinity, of course there are infinite different scales of infinity, you could say that 10 divided by zero is 10 times as much as 1 divided by zero, but both are errors. You listed specific integers, but didn't specify real numbers, or integers, you just said numbers. I used a slightly liberal interpreation of number to imply anything used to track a value, and took the opposite... its really more of an antinumber. But, I voted for 0, since 0 was an option, so, if null is not an allowable answer, then, you already got 0 from me. I hope 0 is enough. If 0 isnt enough, then add 0 more to reach the same amount of not enough.