answer.
I'm not sure how many different copies you can have in parallel or if there even is a limit. I run it on two machines and it is no problem. the one still has the beta 3 patch, this one has 1.10. so, from experience I'd say it should not be any problem.
I once heard you can have two copies at any time and that you can use them for lans too. I think it was done to encourage spread and make it easy to play with a friend who may then proceed to pick the game up. at least I have been able to play with my single copy, legally, with a friend of mine. he eventually got the game. 