For anonymous (not authenticated) the sku should be something like "Power Apps Portals page view capacity add-on" believe it is for 100K views. I believe it is a page view i.e. navigate away and then go back to the same page count would be 2.
Each unit provides 100,000-page views per month. For example, a customer requiring 570,000-page views per month, should purchase 6 units of “Power Apps Portals page view capacity add-on” - as per the PowerApps licencing guide you need to anticipate your max anonymous page views (it does not auto bill at the end of month based on actual).
I am not sure exactly what happens from the portal users perspective when you exceed the page view limit - until recently Microsoft were not enforcing some of their licencing, but they have recently started enforcing.
note: this licence is not for the users that are authenticated external users (i.e. logged in) or internal users.
