[barkerja]

Multiple passkeys backed by different sources (password manager, iCloud, Yubikey, etc.) can serve as a backup in the case you lost access to your password manager, for example.

If a service provides the option for more than one passkey, I always configure several.

[lolinder]

But that doesn't explain why you'd want multiple keys all in the same password manager. That seems to miss the point of the redundancy, like keeping an "offsite" backup onsite.