I am not with Laminar Research; just a flight simmer.
You can install and run a single legal copy of X-Plane on any number of computures as you like. However you cannot run multiple computers with the software at the same time.
If you have a disc copy, disc 1 has to be in the DVD drive of the PC you are running at all times. There is no USB dingle as a substitute for disc 1 as was the case with XP10
If you are running a digital download you have to enter the password key and be connected to the internet at all times. My understanding is the software performs a "secret" test to determine if your copy is legal. To confirm the legality of your software you may be asked at different times to re-enter the password key.
Bottom line, if you run more than one copy at any one time you need legal copies for each version.
My version of X-Plane has been the disc version. I am not aware of any differences between the Mac version and windows version for a digital download.