Well, if you were to install Win2k on, lets five different computers and one of those installations failed after and had to be reinstalled. You are correct, M$ does not know how many installations but, when each computer hits the Internet it will phone home and report that it has been installed. The machine that needed the reinstall may be reported as two installations. Remember, Windows is designed to grind to a halt unless you get those updates from the M$ site.
After Uncle Bill's folks gather up all that information they need only charge you for five copies of Win2k. Provided you have registered the software to cover the first installation.
If M$ is kind enough to set up a system where you pay them yearly for software licenses (or monthly?). Then it could be set up through the bank for auto payment, just like the electric company or phone company.
That way when your computer crashes and you have to reinstall Windows or when you go to the M$ site and download all the patches needed to keep it from crashing. At the end of the month M$ will just take what they think you owe them for the privilege.