Counta:
Counts the number of cells that are not empty and the values within the
list of arguments. Use COUNTA to count the number of cells that contain
data in a range or array.
It depends on whether you want cells with a "" response to be counted or not
(they look blank, but have a formula in them)
"" counted try =counta(D)
"" not counted try =65536-Countblank(D)