I don't think you understand how FIDO2 works. If you are using non-discoverable keys, the type where you input your username to login and then it asks for the

I don't think you understand how FIDO2 works. If you are using non-discoverable keys, the type where you input your username to login and then it asks for the passkey, there is no limit because they are calculated from a single master key and seeded with the url. If you are using discoverable keys when you don't even input a username to login, the latest Yubikey has a limit of 100 but other vendors hold more. Physical hardware keys work across most devices and browsers as long as you have the key with you.