There are 'portals' and then there are 'portals'. That is my takeaway in how the term is used from a licensing and from a technical perspective.
From a licensing perspective, one gets one free portal and then can buy additional portals if desired.
Many interpret this to mean that they need to buy a portal license for their Customer Engagement type portal and then more for each other portal they wish to deploy. This is not correct. One doesn't need to buy further portals to host different portal types.
When you get a 'portal' from a licensing perspective, it means you are effectively getting a single application web server to host any number of portal web sites within a single environment in a single Dynamics 365 tenant.. (Note: with recent changes, one doesn't even need a Dynamics 365 tenant but just a PowerApps Portals!) As an example, you can host a custom portal that acts as a Content Management System (CMS) to your corporate web site and a customer engagement portal for managing external partners.
The point is not to confuse the free 'portal' application web server license with the 'portal web sites' that you create within Dynamics 365 (or PowerApps Portals).
Now you may draw the conclusion that you don't need to buy additional portal licenses. You can certainly get buy without buying additional ones, however...
...see my next post on portal release best practices.