According to this doc All You Need To Know About Passkeys at Auth0! each user can have up to 20 passkeys.
Universal Login will only prompt the user to enroll a passkey once which makes sense but what if they want to enroll another passkey?
hey there 
just to understand the use case better, are you looking for an additional passkey from within the same device?
for new device… let’s say initially the user enrolled using macos then want to enroll another one using their android device