I'm not sure how it works now, but a few years ago it was illegal to use more than one copy of Windows in this way (provided that you already have a purchased copy). If you wanted to have two instances, you had to purchase a second copy. I'm not sure what the relevant terms are for macOS now apart from the fact that in the old days VMware had an artificial block you need to remove in order to run Mac OS X. It would be interesting to know the current legal ramifications.
AFAIK you can use Windows unlicensed as long as you activate it after evaluation. You're right that you can only run one instance of a given Windows product code on a machine for long-time use, though.
The last I heard about the macos situation is that virtualisation of macOS is allowed as long as you do it on Apple hardware that you have a license to run macOS on. You can run as many virtual machines on your mac as you want, as long as you've paid for the OS on your mac, which you can't buy separately anymore (making any restrictions on licenses a full ban on macOS VMs, which nobody wants). Perhaps you could buy additional licenses through second hand shops, depending on your jurisdiction.
As for legal ramifications, I don't think either Microsoft or Apple will care if you use this for your personal projects. Don't try this as a business, though, because that's where the lawyers start caring.
Legally, you'll probably be liable for a civil lawsuit from either Apple or Microsoft. I don't think it's actually considered a crime to run a copy of software without a license in most countries.
> You can run as many virtual machines on your mac as you want
No, the EULA says:
> (iii) to install, use and run up to two (2) additional copies or instances of the Apple Software
within virtual operating system environments on each Mac Computer you own or control that is
already running the Apple Software, for purposes of: (a) software development; (b) testing during
software development; (c) using macOS Server; or (d) personal, non-commercial use.
Interesting. I was always under the impression that you can run a macOS VM as long as it is on Apple hardware. But this says "that is already running the Apple Software".
Does that mean Apple Hardware -> Linux -> macOS is not allowed? Or is it simply saying that all 3 copies must be on the same machine? So you can run Apple Hardware -> Linux -> macOS as your "primary copy" and two more macOS VMs on that same Apple Hardware?
IANAL, but this whole section of the Big Sur license is a bit of a mess to read and understand — for example, per section 2B(i), copies of the OS downloaded from the Mac App Store by individuals may only be used for "personal, non-commercial use", and downloaded from the MAS only on computers running older versions of the OS.
In other words, for an individual (vs. a "commercial enterprise or educational institution"), both of the following appear to violate the license:
(1) Downloading Big Sur from the Mac App Store using a machine running Big Sur itself, for any purpose: the virtualization section 2B(iii) expressly excludes the term "download" from the phrase "download, install, use and run", and, while the other section 2B(i) applicable to individuals does permit downloads, it only applies to computers running Catalina, Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion, or Lion.
(2) Using Big Sur for any commercial purpose, e.g., working from home as an employee, or non-personal purpose, e.g., producing flyers for a school bake sale.
The old trick of buying an OEM copy of Windows (instead of paying full retail) when you were not an OEM building a computer to be sold to others was just as "illegal".
