I have two gmail accounts, how many FIDO keys do I need to protect two gmail account. Do I need 2 FIDO keys?
You can use one FIDO key with multiple accounts, but that is not recommended. Regardless of if
you're protecting one gmail account, or two - you should register multiple keys (if the service allows it).
That way, if you lose your key, or it's damaged, or it's stolen, you will still have a backup key ready to use.