On a Mac, you can run Boot Camp, requires you to have a copy of windows to install on the partion that you're creating for Boot Camp tho.

And yes: you need to buy a license for every platform you want to play on. I have for Win myself, and only ARR on PS4 as I didn't play too much there after all.
I was considering the Mac-client myself as I figured it would be better to play on the native client, but all the user feedback sugest that you will play better under bootcamp, as they didn't make a native client for mac, they just framed it in a built-in WIN-emulator of sorts.
Hopefully they will give some information on the live letter next week.
But for now: Bootcamp is the way to go, then you can still use your windows copy of the game