You may buy the multi-platform version of Lightroom 6 so that it can be installed on both Mac as well as Windows.
If you will go for only windows platform version, then it would not be compatible with Mac and you will have to go for platform swap later.
And if you are planning to for the creative cloud subscription for Lightroom, then you don't have to worry about it as it is multi-platform and is compatible with both Mac and Windows.
Let us know if you have any further queries.
Yes you can. You are permitted two activations. Windows or Mac or one of each. If you buy the boxed version you get disks for both systems and can still use a download to install.