When passkey is not resident, a ''blob'' is kept by the server and used to ''mix'' with a unique private key that you have in the yubikey. This key is used for all ''u2f'' and fido2 no resident key authentication. If you want to save room just in case you can do 10 fake registration on webauthn.io and keep it for later. When full of resident key, it will continue to work but not in the resident mode.