Seems to work. Nice one!

Thanks - a "trick" that occured to me is to use negative values.
This simplifies things as it puts the frequencies in the relevant
rows, and it shifts the zero count past the last row which means
you don't need the IF statements. This should allow for non-array
formulas but i haven't found a simple way to do it.

Also, with unconcatenated data in B2:C12 you could try {array-entered}:

=COUNT(1/(B$2:B$12="A")/(C$2:C$12="C")/(FREQUENCY(-ROW(B$2:B$12),
-(1-(B$2:B$12="B")*(C$2:C$12="D"))*ROW(B$2:B$12))=D2+1 ))