Adding count of unique values in a pivot

=COUNT(A:A) on the pivot sheet?

Your profile says you're on Office 365, so you may already have the Dynamic Array functions, currently rolled out to users on the Monthly channel. With Dynamic Array functions, you can use the Unique() function to get a count of unique values, for e.g.

=COUNTA(UNIQUE(Data!A2:A25))

Or, if you add the source data of the pivot table to the Excel Data Model and build the pivot table from that, you can use the Distinct Count in the Value Field settings.