Will a Microsoft account save my windows 10 activation key?

will signing onto my microsoft account on windows 10 save my activation key for future use with a new pc?

For future use with a new PC? That depends how you obtained your license, but it's unlikely to be transferable.

Ms stores details of the pc hardware and you ms account so a new pc wont match the hardware so you should buy a new win 10

Read this information closely:
https://support.microsoft.com/en-us/help/20530

Superfly said:

Your MSA links to this but as before only Retail activations are transferrable.

oem keys can be sold on in the EU but not in the states - which then infers if you can sell on keys that you own you can then can transfer them on the machines in your posession. This is partly why so many (adobe) moved to subscription services, monthly revenue from subs is far better as a business model than one off payments (slight digression)

You can transfer a retail licence between PCs - but the old PC is likely to deactivate. Always keep hard and soft copies of your licence keys for future use.

Steve C said:

You can transfer a retail licence between PCs - but the old PC is likely to deactivate. Always keep hard and soft copies of your licence keys for future use.

The old PC will not deactivate with Windows 10 new digital licensing system. Once a computer gets a digital license for Windows 10, it has it for life.

NavyLCDR said:

The old PC will not deactivate with Windows 10 new digital licensing system. Once a computer gets a digital license for Windows 10, it has it for life.

What happens if the old PC originally had a retail licence? You are allowed to transfer that licence to activate a new PC. Your reply implies two PCs can be activated via the same original licence.

Steve C said:

What happens if the old PC originally had a retail licence? You are allowed to transfer that licence to activate a new PC. Your reply implies two PCs can be activated via the same original licence.

Nothing happens to the old PC. Scenario:
Joe buys a new PC, installs Windows 10 Home with the retail license from his old PC. Joe wipes the old PC drive and sells it with no OS.

New owner pops in a Windows 10 USB flash drive and installs Windows 10 Home with no product key. As soon as the old PC connects to the internet, it will retrieve its digital license stored on Microsoft activation servers based on the computer's hardware ID (not product key) and will activate.

There is nothing a consumer can do to remove a digital license stored at Microsoft, and Microsoft is not actively removing them either. However, by removing Windows from the old PC, the consumer has done all that they can do to comply with the EULA.