I am making a hamburger and hotdog cakes for a Labor Day party and I want to make sure I have enough.
The hamburger will be...
Top bun - 8in cake in a Pyrex bowl
Burger - 8in cake
Bottom - 8in cake

The hotdog will be...
A loaf pan with the top center cut out for twinkies/hotdog.

Thanks for the help.

you can always make an extra cake and use it as a bun in the middle, or a double cheeseburger with two patties, like the one i did here:

http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=754903

From your description I'd say you will be using 3 mixes for the 'burger so that should serve 35.
My guess on the loaf is you used 2 mixes so there is another 25 servings. That's based on servings approx. 2x2x2 OR 1x2x4.