Thank you for posting your query on Microsoft Q&A, from above description I could understand that you are looking for advisory on billing of Microsoft authenticator app.
Please do correct me if this is not the case by responding in the comments section:
As per B2C Verification methods SMS/Phone-based multifactor authentication incurs separate charges from the normal Azure AD B2C MAU's pricing model.
Microsoft Entra B2C billing is done on Monthly Active Users basis (MAU) under which First 50,000 MAU authentication are free including the MFA via Microsoft Authenticator App. Post that tenant is billed on €0.002960/Monthly Active Users for P1 and €0.014799/Monthly Active Users for P2. If using SMS/Phone for MFA then it adds up cost by €0.028 for each SMS/Phone-based multi-factor authentication attempt (not Microsoft Authenticator).
Thanks,
Akshay Kaushik
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