I was applying for class action money from Microsoft at http://thatsuitedeal.ca/ and I had to mentally audit my OS licenses over the decade or so involved. I was a bit shocked when I realized how many Windows licenses I have purchased over the years. I still have at hand my 15 or so various licenses of Windows XP, Vista, 7, 8, and 8.1. My 2 Vista licenses are still new in box, having never been installed successfully. (I built a machine back then with no floppy drive, and I therefore couldn’t boot it off CDROM without the CDROM going away in the process, so I never could get it installed. I ended up sticking on Windows XP until Windows 7 came out, containing the necessary drivers.) The downside is I own no actual Windows 10 licenses (just entitlements). Luckily, because of the way they’re managed now, I can enter an old license when installing a new machine so I don’t have to first install an ancient OS (7 or 8) and then “upgrade” to 10.