You must have a unique Product Key for each machine. No one here will assist you in circumventing this.
If you use the same key for more than one machine, the first machine that Product Key is used with will activate and be legitimate. Every subsequent install on different machines will result in the OS not activating. While it will work (with some limitations), you will be constantly nagged that the copy of Windows on those machines isn't Genuine.
Technically you only need one install media (USB / DVD) as the Product Key isn't tied to the media itself. So you can get one USB key (or buy an 8GB key and use the Media Creation Tool), then purchase all the Product Keys you need online.