It's not much of a hassle. I'll add at least two when I want to start using a passkeys for a site. So maybe add a passkeys to the phone and to my keychain device. Then, next time I use the service on my laptop, I'll sign in with either my phone or keyring (whatever happens to be closer) and make one there. Then next time I want to use the service on my desktop I'll sign in with whatever I've got nearby and add my desktop and the token in my desk drawer. And maybe my password manager also has a passkeys, added somewhere in there.

It's not like every time I sign up for a new site I have to drop everything right at that instant and go add a passkey to every single device I own.