There has never been a specific number given. The EULA allows you to install it on as many computers as you wish, as long as you are the primary user of all of them and you won't be using it on more than one at a time (except for the network rendering client). However, if you register it too many times or too rapidly the registration server will put a block on your serial number. This happens very rarely though (only once to me so far) and can usually be solved with a quick phone call to customer service.