It's officially per-user, meaning you can install it on multiple machines that _you_ own and run it individually or concurrently across all of them for yourself only.
Although I think it would be in the spirit of the license to satisfy *either* of the criteria-- either install it on only one machine (for you and your family to use) or install it on multiple machines for yourself only.
Note that I don't speak for Khaled here, but that's the general/unofficial feeling I get on the subject.
