Hi,
I'm Dyari. Thanks for reaching out. I will be happy to assist you in this regard.
Let me clarify it for you briefly.
Activating Windows after hardware change depends on the type of license you have.
If your computer came preinstalled with an OEM license, the license was tied with your old motherboard and it would not be transferred. You should buy a new one.
If you bought the product key separately and it was a retail key or if it is a digital license linked to your Microsoft account (as yours), it should activate the Windows automatically after signing in with your Microsoft account as long as you install the Windows edition that matches the license type. (if you have Windows 10 Pro, you must install Windows 10 Pro on your new computer).
If does not work, try to troubleshoot it by going to Start > Updated & Security > Activation > Troubleshoot.
Based on your explanation, you've performed an upgrade previously. I was wondering whether you upgraded from Windows 8 to 10 or if it was a hardware upgrade.
If you did a hardware upgrade (only the motherboard) and you were able to activate your Windows, you should be able to reactivate this time too.
And yes, only one device can be activated at a time.
So, if you still have access to your old computer, kindly click on Start > Run CMD and type the following command then hit enter:
slmgr.vbs /dlv
A dialogue box should appear > Take a screenshot and post it back to see the type of license you have.