When I purchased LR4 in April, I thought, I could install one copy on up to two computers that I owned (my desktop & my laptop). Tried to install my licensed copy of LR4 this morning and was asked if I wanted to buy a number to install LR4 on my laptop, what gives? Did I miss something!
Just enter your serial number (didn't it ask for one)? There may be a fixed number of installs the EULA allows, like two computers are specified but I don't believe LR, like many other Adobe products tracks this with activation or anything like that. The licence states what you can legally do, you should of course follow this. But I don't know you are forced to.