Microsoft sells a traditional single version license (i.e., one-time price, no recurring subscription, no free upgrades to Office 2017 or whatever comes next) for a single Mac.
They also sell an annual subscription for less, which includes upgrades to the next major version(s) as well as license to install on up to 5 Macs/PCs plus 5 tablets/phones. The catch though is you must pay the annual subscription fee or the software will lock when it expires.
Highly recommended, gives you 1TB of online OneDrive storage as well as being able to install Office products on 5 computer both Mac and PC, not to mention use iOS/Android versions of their apps.
