I want to get into building computers but I don't want to have to buy a new copy every time I build a computer.
https://www.windowscentral.com/you-do-not-need-activate-windows-10 describes a process whereby you
- download Windows from http://www.microsoft.com/software-download/windows10 (which lets you download an ISO if not running Windows, the media creation tool if you are),
- create installation media (USB drive, DVD, or ISO, apparently), and
- install Windows without entering a product key.
According to the article, this gives you a functioning copy of Windows for a limited time (article says at least a year, commenters on https://superuser.com/a/1316305 say 90 days).
If you plan to use the computers yourself, you'll therefore soon need to buy licences.
If you plan to disassemble them and rebuild them in different ways, you should be okay just blowing away the installs and installing again, I would think.
If you plan to sell your experiments, I'd probably buy licenses, as I imagine your buyers would be unpleasantly surprised to find they weren't included.
As commented above, not legally.
What you need to buy from Microsoft, after qualifying as an OEM, is media for OEM installations, and a block of license code stickers. The media is able to install (way faster than a regular install; essentially in a single pass) the OS partitions and the recovery partition for the block of license codes. Each installation is coded to a unique license code and the associated license code sticker is intended to be adhered on the computer/device.
The license codes are registered with Microsoft so that, when required, the OS installation can be be validated by Microsoft, when the computer/device is for example first started up, or perhaps later during an update.