Multiple physical keys in multiple formats stored in multiple places. I've got a few security keys: one on a keyring, one in a locked file cabinet, one locked in a safe. Backup codes are printed on good quality acid-free paper with good toner and then are put in acid-free envelopes or laminated, stored in a safe at home and a trusted family member's house.
I'm far more concerned with an attacker from the internet or destruction from a fire or natural disaster than someone using my computer at my home who happens to have my username/password combination as well.
I have two. One stays with me, the other in a safe.
Which is legitimately a pain in the ass to register, manage, keep in sync, etc but I only use it for a few very important accounts--Google being one of them.
You should have emergency backup keys as well printed on real paper. I have a set stored with our important files, and another in my nightstand. Having my phone stolen would be a damnable inconvenience, since that is my second factor -- although not via SIM, but via Google's in-app authentication. But it wouldn't be fatal.
I use andOTP, which is compatible with Google Authenticator and actually allows backing up the private keys (one of the key missing features of Google Authenticator, though maybe they've fixed that).
