The liscense agreement as far as I know is for one install. However many simmers do use it on more than one computer. I have had to call in the past as this last two years has been a bad time for computers and operating systems. A friendly voice on te other end of the line, Floyd or Julie in Mumbai... They will keep asking you the same questions, hoping or a misstep.
I do have two copies, one a purchased version and the other the RTM build from the end of beta test.
Cheers: T.