All of Patrinicia’s questions are good ones and the same I would ask.
In the Cake Bible, Rose recommends an 18x12” sheet pan for 80 people, but if your couple wants a cake with tiers, I think two round tiers (the bottom using a 11 or 12” cake pan and the top using a 7 or 8” pan) would be pretty and plenty of cake. For three tiers, you might want to do a 4” top tier, 7” middle and 10” bottom. According to Rose’s charts, you would be left with a extra cake.
I did a 10” bottom tier and a 6” top tier for a wedding of 50 people, which was supposed to serve 75, so I thought I was golden.
But the people cutting weren’t caterers (who are used to making small “wedding~size” slices) AND they were hungry because they were only served appetizers so people got hefty slices of cake (which was already a heavy chocolate torte with raspberry buttercream and chocolate ganache!) and the cake was GONE.
I don’t mean to make anyone else planning to bake a wedding cake worry, though…..If the wedding is catered, the caterer will slice the cake correctly and make sure there is plenty. It’s not typical to run out of cake.