If i buy the PC version can i play on mac too? or do i need to buy a 'key' for each platform?

Yes you can, as long as you remember to unregister before you try the mac version.

You need a key for every copy of the game you want to register.

At the same time. You can unregister your PC version and then register your Mac version, or vice-versa, every time you want to switch. If you want both to be running at the same time, you do indeed need two keys.

But Importantly the point is as long as you Unregister when you stop playing your PC version the key will work fine on your Mac version and vice-versa. Ignore BladeCat, he's been complaining about that for a while and refuses to accept that there's the ability to deregister to play on a different platform...