Well, here is my situation:

Would I have to buy cortex command once or twice, since I use both Mac and PC.

It is going to be used on one computer. That one computer has Mac and Windows both on there.

Does one physical computer with two OSes count as one computer or two computers?

And are the keys for Mac and PC compatible?

I use both operating systems equally, and I honestly don't feel like restarting just to play Cortex Command.

Thanks.

EDIT: Can I also buy a license through snailmail?

I have no idea, but if they are on the same comp, maybe.

Email Data about it.

I would actually think not, at least with the current system. However, there's no telling whether Data will add a system specifically for people in your situation. Also, I seem to recall him talking about remote unregistration as part of B21; as in without having to email him. That would solve the problem anyways.

Afaik the key system is completely conprehensive, unless there is a "buy for mac" button, I am right.

Just unregister it every time you switch OS'. Hurr.