According to what i could find on wilton.com a 10in, 8in, and 6in wedding cake will feed 74. Is that right... kind of sounds a little low. How many servings do get out of a cake that size??
It depends on what shape the pans are, and who cuts the cake.
Wilton's Wedding cake chart is a little stingy, and the party chart is a bit generous. I pick a number somewhere in between.