Which key is Num Lock on an Apple keyboard in Windows?
Under Boot Camp, it's the Clear ⌧ button, according to Apple's documentation.

That makes some sense, at least, because that key is where Num Lock normally lives on other keyboards.

(Note: I don't have the same keyboard to test with, but I'm guessing this document isn't actually Boot Camp-specific; it's just a guide to how the keys are mapped under Windows.)