Try the below code in UI.
If( IsNull( GetCurrentSelections()) ,0, GetPossibleCount(DimCustomer.CustomerID))
If you do selection in any field and the Data got filtered in CustomerID as well even though you didn't selected the CustomerID , By using this Expression you can get the Current Selected count of Customer ID .
I also have a question related to getfieldselections() :
I use this function in a chart expression: the user selects the months from a listbox and the expression sums the sales by taking only the months in the table that are equal to the selections.
Anyway, I noticed that when selecting more than 6 months the current selections box shows "NOT Aug,Sept,...". Because of this result I can't get the selections.
Anyone has an idea on how to resolve this?