I have to make a cake to feed 100 people for a wedding. How many tiers would the cake have to be to feed that many?
It's not so much how many tiers it takes as it is what size those tiers are.
Use the Wilton wedding cake chart to determine what size, shape and how many you need.
The industry standard of a 'serving size' is 1x2x4, which is what Wilton's chart bases their #s on.