[AlexandrB]

The Yubikey is the "one device". But most people don't buy Yubikeys so the "one device" is, in practice, a smartphone.

[Telaneo]

Even then, unless you use one for work, where work can issue you a new one if you lose it, you're going to need (at least) two Yubikeys if you want to go that route, because not having a backup is a bad idea.