I bought mine from Amazon - the listing says "The number of computers eligible for installation may vary". I'm almost certain the install process said I could use it on 5 PCs, but as noted above, that may vary with vendor (it's marked "Amazon Exclusive"). I've only installed it on one so far.
Did it prompt you to enter the authorization code multiple times, as in the link above? If so, that's a known issue.
Did the listing where you purchased it state the number of installations?
