> How do you deal with backups when using YubiKeys

You generate your gpg key on a computer, back it up securely, then store it in the YubiKey and delete the unprotected version on the computer.

This allows you to have multiple YubiKeys with the same private key.

However, nothing prevents you from generating private keys on the YubiKey and using multiple Yubikeys. Or doing a combination of the two approaches.