Yes google does.

The spec strongly encourages providers to allow multiple keys, and allow you to nickname them.

As far as I know everyone allows as many keys as you like except Vanguard and Amazon AWS (which both also only accept Yubico keys)

> Does anyone know if Google allows registering more than 2 keys? Like what if i want to register 3 or 4 keys for advanced protection?

Most sites do. Google and Github both allow multiple U2F keys (note that this means any key can be used to unlock your account, not that all are required).

Of all the large and popular sites that support U2F, the only one that I'm aware of that only allows one key is Twitter.

Yep, they do. I’ve registered 6 without a problem.

It’s cheap insurance against getting locked out of my Google account forever.

Sure but you're diluting the second factor. The more tokens you have, the easier it is to get one and without you noticing.

There's always a tradeoff between convenience and security, but I'm relatively unworried about someone, say, breaking into my bank compared to someone spoofing my phone number.

Just to be clear: it only requires ONE key to be present. Supporting multiple keys is for BACKUP, not additional layers of security (if you were thinking of distributing two keys, one to each person, to make it required for both parties to be present to auth).

Yes no problem