@Mr-Taco Here is an article from Chris at Microsoft that covers reimaging rights. https://community.spiceworks.com/how_to/124056-reimaging-rights-for-windows-10-licensing-how-to
I can tell you that if you have 100 Window 10 OEM licenses, you only need to purchase (1) Windows 10 Pro VLK key to deploy to all 100 of those Windows 10 Pro OEM machines. That rule only works for the same version. You can not purchase 1 Windows 10 Enterprise VLK key to do a version upgrade to those 100 Win10 Pro OEM machines. For a version upgrade you need a 1 for 1 volume key. So 100 OEM would require 100 Enterprise keys.
But back on the 1 Win10 Pro VLK key, you can purchase that key but you need to get a count of 5 so people will often purchase Win 10 Pro VLK key and 4 windows (4) server CALs (or something else cheap that you can’t get enough of anyway) to get to your count of 5.