[luhn]

If you have one Yubikey, how does system know if it’s really you or someone who stole your key?

[LeonM]

It is a second factor, so the'll also need to know your password.

You will notice your key missing, then you can disable that key with your backup key. With only a password, it becomes a lot harder to notice someone stole your pw.

[icelancer]

Right. That also applies to N number of keys, too.

[joesb]

The different is, if you have 1 U2F key, people who steal your U2F key gain access to one half of the two factors for ALL services you use.

With 4 U2F keys, people who stole 1 of your U2F keys gain that one factor for only the services that you tied to that keys.