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With Windows 10, the device limits have been reduced to 10:
https://www.thurrott.com/windows/windows-10/4959/ask-paul-what-are-the-windows-10-device-limits
But Digital Licensing is not attached to the Microsoft Account, it is attached to the hardware with the qualifying operating system.
When you upgraded from a previous version of Windows or receive a new computer preinstalled with Windows 10, what happened is the hardware (your PC) will get a digital entitlement, where a unique signature of the computer will be stored on Microsoft Activation Servers. The Windows 7 or Windows 8 genuine license you were previously running will be exchanged for a diagnostics key.
Anytime you need to reinstall Windows 10 on that machine, just proceed to reinstall Windows 10. It will automatically reactivate.
So, there is no need to know or get a product key, if you need to reinstall Windows 10, you can use your Windows 7 or Windows 8 product key or use the reset function in Windows 10.
If you are prompted to enter a product key, click 'I don't have a key' and 'Do this later'
So as long as those systems are activated with Windows 10, Microsoft Account or not, it earns a digital license.
More information:
For anyone else, including those with local or domain accounts, this step is optional. In any case, it applies only to those who have a Windows 10 digital license. That group consists primarily of those who took advantage of the year-long free upgrade offer that ends on July 29, 2016.
This new feature doesn't change the fundamental way that Microsoft's activation servers work. The process of activating Windows relies on a unique installation ID, which is based on a hash of information taken from the hardware on which Windows is installed. That hash is reportedly not reversible and is not tied to any other Microsoft services. It identifies a specific device, not a person.
