You can install X-Plane on multiple computers with either version. With the digital download version, if you need to make a copy of X-Plane on a second computer or second hard drive, we recommend you copy your current X-Plane folder, rather than installing a second copy.
If you have multiple people who want to fly at the same time, I would recommend buying each one their own product. Our DRM might lock your product key if it is used on more than one computer at the same time--it looks like someone stole your key.
You can install the DVD version on as many computers as you like, but you will only be able to use one at a time because you must have the DVD #1 in the disc drive to get out of demo mode.