not sure if I understand correctly what you are trying to achieve with aggr, but when you simplify the expression to "Count( DISTINCT PersonID)" it seems to work fine. if you want to then see the zeroes for the products where there are no data points, just untick presentation --> suppress zero-values.
is this what you were looking for?
in count actualy I want the number of all doctors in that specialty. like in gen there are 4 doctors and in ENT 5 and 4 should not change even if we select some product.