You can add multiple passkeys on your Google account today. If a website is dumb enough that they only allow a single passkey per account, they have misunderstood passkeys profoundly.
Google != passkeys. Google may allow multiple keys, but that is outside of what FIDO specifies for passkeys.
Here's a source saying that passkeys expect one and only one key:
"Passkeys are designed to [...] allow the FIDO credential to roam across multiple devices. This [means that there's ] no need to repeat enrollment on each device [.]"
> If a website is dumb enough that they only allow a single passkey per account
