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Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client and know little about Microsoft Authenticator. The following is for informational purposes only.
Based on the information I searched, an account can only set up Microsoft authenticator app on 5 different devices. As an alternative, you can use security information such as text messages, phone calls, or security keys as a method of two-step verification. You can log in to the Security info page here to set up: https://myaccount.microsoft.com/.
For more information about two-step verification, you can refer to: Sign in using two-step verification or security info - Microsoft Support.
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