As far as I know, there is not a serial # or key. I recall when you create an account using software from the retail disc, the subscription and updates are marked as a retail in your account.
Out of curiousity though, how do they prevent people from simply creating a new account or something along those lines and being able to install it on 3 different PCs?
Unfortunately, for security reasons, I cannot disclose any anti piracy protection methods. You may want to try McAfee's official customer service to see if they can tell you more.