You're not limited to a single passkey by the spec or 99% of sites that support them. The limit is arbitrary and typically when a limit exists it's no fewer than 3 (very rarely 1, but I've only seen that once that I can recall).

I generally add three, one for Bitwarden (my actual password manager), and then the OS passkey store (Android/Chrome password manager or iOS/Apple Passwords depending on the device).