1. Yes! Using resident keys, yubikeys can store a number of them. Not an infinite number. It's 25 resident keys.

2. Most services let you add more than one passkey. Using 1Password, or using iCloud Keychain or similar, you can sync passkeys between devices. Even with iCloud Keychain, if you have only one device, you're given a recovery code that can bootstrap the entire system from zero if your only device is stolen.