Portal requires an endpoint to fetch the total number of unread user notifications. Today the Portal behavior is odd without such an endpoint, it has to fetch pages to discover the total count.
In azure/dev installation running pm-1.16.1 when I first visit or refresh the Portal home page the bell icon in the top right shows that I have 5 unread notifications. After one second that number increases to 19. After another second that number increases again to 25. Then the number stays fixed at 25. This isn't harmful but it's a little disconcerting. Possibly you are fetching one page of notifications at a time from CDS and reporting the aggregate count for this?
I suspect this can be best solved by providing a CDS API to retrieve the unread notification count for a user. That API is not currently available, but it would be very easy to create it. Please advise.