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Microsoft specifies that up to five MFA tokens can be associated with one account. The limit applies to hardware and software OATH-TOTP implementation including Microsoft Authenticator apps. For example, you can associate three YubiKeys, one Microsoft Authenticator app, and a phone number to an individual account if no other OATH token is being used.
If you have questions about using your Yubikey for Microsoft MFA, please contact the ITS Service Center at ITS.ServiceCenter@fredonia.edu or (716) 673-3407.