all you need is a disc of whatever version of windows, and the required key for it. with that, you can install whatever version as many times as you want. like, this isn't a hard concept to understand, nor is it even a new thing. it's been this way with every single version of windows
no... you need 1 license per computer that you do this to.
you can't (legally) take a windows 10 disc and install windows 10 on a dozen windows 7 computers. that would be illegal without purchasing licenses for windows 10 for each of those computers. (yes I'm aware that you can still do it illegally.)
however, you *can* take a windows 7 disc and install windows 7 on a dozen windows 10 computers. that is perfectly legal even without purchasing licenses for windows 7 as long as the computers already have windows 10 licenses. this is called downgrading windows, and doesn't work for every combination of windows versions. you can see the details here: