Many services which support passkeys do not support multiple passkeys. The big ones have figured out that we need support for multiple, but sadly just like with Yubikeys not everyone got the message on that.
But, its like I said: You have zero concept of the struggle that is already happening and will continue to get worse. That's what I mean when I say that the software industry has zero concept: You think you have the solutions, but you actually haven't even grasped the full scope of the problem.
This. Almost no-one's even realised that maybe being able to enroll multiple TOTP tokens would be a good idea. I spent some time recently trying to figure out a good, properly secure, robust authentication method would be for various important accounts both personally and at work, and I've more or less concluded that there isn't one, the best compromise that mostly works is TOTP but manually copying the secret across two hardware keys, which is wayyyyy outside the average user's capabilities, and I'm not even sure I'll be able to make it work in practice.
passkeys are not serious until they actually address backups in a way that isn't just "we'll copy the secrets around in our cloud services just like passwords lol"
(And it's not like there's no solution here: firstly make it mandatory in the spec to allow enrolling multiple keys, then standardise a means to enroll a device from another device, automatically, across all devices that other device is enrolled in, and then also it would probably be a good idea if that also offered a way to revoke the other keys)
The only site that uses passkeys I regularly use that doesn't support multiple is PayPal. Pretty much every other site that at one time only supported one now supports multiple. Which common sites still only allow one, other than PayPal?
But, its like I said: You have zero concept of the struggle that is already happening and will continue to get worse. That's what I mean when I say that the software industry has zero concept: You think you have the solutions, but you actually haven't even grasped the full scope of the problem.