AHow many slices do you get out of a cake that is shaped like a #1! Here is an example: http://www.birthdaydirect.com/number-one-cake-pan-p-12233.html
AI would estimate that pan has a total surface area of about 8"x14". A single 2" layer would give a volume of 224 cubic inches. Party-size servings are 12 cubic inches, so that works out to almost 19 servings for a single layer.
It says it takes a single box of cake mix, so however may servings you'd usually get out of one box of mix I guess, that depends on how you serve your slices!