I'd use a helper column and put this in the top row of the data:
=INT((ROW(A1)-1)/8)
And drag down.
Then you can subtotal your data by that column.
You could also put:
x in the first 8 cells
y in the next 8 cells
then select those 16 cells and drag down the column to fill it with groups of 8
x's, then groups of 8 y's.