With keys such as a U2F key, you can use it for multiple sites (each site will have a site-specific keypair). Most sites allow you to associate multiple U2F keys as well. Since U2F keys typically cost 7 to 15 Euro, it's really not much of a problem to buy one for use and to put one as a backup in a fire-proof safe.

That said, I think it's bad to rely on one factor. If the key gets stolen, security is compromised. The combination of a password and a security token is far more secure. People should just stop memorizing passwords for every site and e.g. use a password manager.