The number of cake mixes required to fill a sheet cake pan is determined by the size of the pan. For example, a small sheet cake pan requires 5 1/2 cups of batter, or one large box of cake mix.
Standard sheet cake sizes range from 7 inches by 11 inches up to 12 inches by 18 inches, and take from 5 1/2 cups to 16 cups of batter.
Some variation exists in the number of boxes of mix required to fill a sheet cake pan. The variation is due to the fact that boxed cake mixes vary slightly in the amount of batter they produce, and egg sizes also affect the total amount of batter each box of cake mix makes.