You can associate multiple U2F keys with a Google account. Since the Yubico's U2F key is really cheap (I think ours were ~15 Euro a pop), it doesn't hurt to associate another key and put it in a safe somewhere.

You generate backup one time password codes at activation time. Store those somewhere safely. Use them in the event of trouble.

Backup codes, or you do what I did: I bought two, and keep one in a safe place.