What licensing is required for enabling Microsoft Authenticator Passwordless Signin?

OneTechBeyond Hi, firstly just to mention Security defaults, this is available to every customer, whatever their licencing and this offers MFA mobile app notification support, while https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/concept-fundamentals-security-defaults#authentication-methods are available with the right licencing. 

To get Conditional Access and the full use of Azure MFA, that requires Azure AD Premium P1, to be compliant, licencing needs to be applied to every user that will benefit from these features. The other options include Enterprise Mobility + Security and Microsoft 365 Business which include Azure AD Premium features.