I am making this cake for my daughter birthday http://www.debbiebrownscakes.co.uk/50_easy_party_cakes/pages/pony_stable_jpg.htm
but I have no idea how many servings it will make?
It is made with a 8" square cake, cut in half and stacked on top of each other!

An 8" cake cut into two is going to make a cake 8" long by 4" wide. I would think you could get 8 slices each 1" x 4".

OR 16 slices if you make them 1 x 2.

The STANDARD slice is 1 X 2 X 4" tall. With square/rectangle cakes the math is easy. length of one side, by 1/2 the length of the other side, therefore, 8 X 2 = 16 servings (also 4 X 4 = 16 servings)