Unlike physical objects they may reside in a TPM, a software vault, an export/backup, or any combination thereof. You may or may not be able to recover or migrate them, depending on where/how they were made.
Therefore you may need multiple per service, or maybe not. Services which only allow one may end up locking you out with no recourse. You get to find out.
None of this is obvious or self explanatory to normies.
As for normies, passkeys or passwords it doesn't make a difference. Either you have people who use love1969 everywhere or those who constantly lose their passwords.
All passkeys accounts for normies require an email or phone number, which is what you can use to recover a password or passkey exactly the same way.
People must understand security controls, at least at a surface level, in order to effectively manage and trust them. Passkeys fail that test.
Passkeys really are not any more difficult to explain than 2-factor authentication. Anyone who’s currently been able to actually create an Apple or Google account and successfully navigate their devices up to a passkey screen will be able to grok how it works.
People around here really ought to stop thinking users are complete idiots. Hell, you don’t even to scroll that far to read people calling users “normies” for crying out loud. What is this? High school?
Roughly one per smart-phone that they've ever used. They don't know the passwords or even the email address of any of them, not even the latest.