Not all passkeys need to be stored on the device. This is exactly what things like key-derivation functions are for. You can have a "primary" device key that derives a Passkey based on the site address with maybe a little salt and/or pepper (but maybe not needed with most KDFs).

FIDO2 allows that just fine, it's a complicated dance between the hardware vendor and the OS right now if your particular hardware device uses a full slot per Passkey or derives the Passkey from some "primary" key.