I have 2 yubikeys and always register them both. So if one is lost or broken then I still have another to log into sites.
As for theft, if you’re using your yubikey as a 2nd factor then they still can’t log in because they don’t know your password. If you’re using your yubikey as both factors then it either has a PIN or biometrics, so the thief can’t log in.
My main use for my yubikey is for SSH. I have PIN-protected SSH with a private key that can’t be exfiltrated
As for theft, if you’re using your yubikey as a 2nd factor then they still can’t log in because they don’t know your password. If you’re using your yubikey as both factors then it either has a PIN or biometrics, so the thief can’t log in.
My main use for my yubikey is for SSH. I have PIN-protected SSH with a private key that can’t be exfiltrated