I have used versions of the basic formula below to count the records
remaining after application of the advanced filter:

=SUMPRODUCT(SUBTOTAL(3,OFFSET(A1,ROW($A$2:$A$19)-ROW($A$1),,1)),--($C$2:$C$1920)),--($C$2:$C$19<=50))

Is there any way to change the OFFSET part to reference the opposite: all
rows HIDDEN after application of the advanced filter? I don't see this
question having been asked before, which is surprising.

TIA