Pre-online registration, there were several different kinds of windows licenses. The 3 main were 3rd party OEM (Dell, HP etc), pc and user. 3rd party OEM is tied to the bios through the service number, so you could change everything except the motherboard and have windows legally. Change mobo's and you called Dell for a fix.
The second, most common license was pc, which was retail, but again tied to the mobo, the biggest difference was upon installing a new mobo, simply required a full reinstall of windows and reapplication of the key that came on the book.
The third was user. This license wasn't tied to the pc, but the user. You could use this key on any matching OS and the number was stored in windows itself. So you could change mobo's and not need reinstall, it'd even boot fine if you stuck the hdd into another OC entirely.
All that said, there's only really one thing any of those licenses have in common. Single use. Only legal for 1 user to use on 1 pc at any given time. No ifs, ands or butts. No exceptions. Perfectly legal for you to move the license to a new pc, under the condition that the old PC is fully retired and no longer valid as a pc. Destroyed. Microsoft allows this to be done exactly 1 time. After that, you'll need to buy a new license.
To run 2 pc's, 3 pc's, 10 pc's means 2x,3x,10x licenses. Microsoft does sell a multiple use license, 3x or 5x, and this is usually cheaper than 3x single use.
Either way, it's still 1 legal license per 1 pc. To do otherwise is illegal and prosecutable, resulting in at least a fine, possibly jail time as well.