License is not shared between Azure AD and the tenant created within this Azure AD

The Azure B2C tenant created under the AAD tenant does not need an AAD P1 license for it’s users, but has a Pay-As-You-Go on a per-Monthly-Active-Users (MAU). AAD P1 can be applied only to AAD users. Azure AD B2C has a free tier of 50.000 monthly users, but if you want to activate premium features like Multi-Factor Authentication, those are billed separately and are not included in the free tier. You can attach AAD B2C service to your application/web-application.

You can create a couple of AAD B2C tenants, and they will be billed based on the number of users and the premium features you activate for it’s users.

The users in AAD are separated from the ones in AAD B2C. For example:

• I have an AAD called Tenant01.com
• in Tenant01.com I have a user called userX (userX@tenant01.com)
• in Tenant01.com I create a B2C tenant that will service the users for my web app called app1b2c.tenant01.com

Based on the scenario from above, app1b2c.tenant01.com is not aware of userX until userX registers as a user in that app with its userX@tenant01.com email.

Only then userX will be a user in the B2C tenant. However, if I activate MFA for userX in app1b2c.tenant01.com, I will be billed for that, even if userX has a AAD P1 in the tenant01.com

I hope my message makes sense.