what size sheet cake will feed 75, a 2 layer 12 x 18 or a single or do i go with a 2 layer 11 x 15 can't decide

How big are the pieces you're going to cut? with square and rectangle cakes, it's easy to just do the math to figure number of pieces.

I go by the standard 2x2 single layer piece. So a 12x18 will be cut in 6 rows by 9 columns (54 pieces). If you do double layers, then I cut them in industry standard of 1x2x4, so you'd cut the cake in 12 rows by 9 columns (108 pieces).

If you plan to cut them 2x3, again it's just doing the math: 2x3 would be 6 rows by 6 columns = 36 pieces from a single layer ... 72 pcs from a double layer.