With Microsoft's usage model, internal licenced users (e.g. Dynamics 365 or PowerApps) are considered as already licenced for the Portal. Other internal users need a licence (even if they do not use the user interface of Dynamics 365 or PowerApps). Note, there is a definition of an Internal User (that is not as clear as it should be in the licencing guide), for the "AzureAD tenant" comment in an earlier reply note there is a difference between AzureAD and Azure AD B2C (but only where the B2C user is not classified as an Internal User - internal user is about the relationship of the person to your organisation not the access mechanism).
External users fall into 2 categories, authenticated and anonymous (both of these have different licencing considerations), number of logins vs number of page views.
Read the current PowerApps Licencing Guide (I haven't linked to it as it changes every month).
