We have Scrivener 1.8 and now that we are using Windows 10 and removing the generic username we have for the computer for security reasons, it seems that the install will ask for the serial number when an user uses their own login.

Is the licensing for per user and not per seat? Or has that changed with 3.0?

I think it’s always been that way, even on Macs. But you can authorize multiple accounts for yourself or others in your household, whether it’s on the same computer or different machines. I think the limit is somewhere north of 5 (maybe up to 10?). Beyond that number, the oldest registration is revoked when you authorize a new computer/logon id to run Scrivener.