I am making a butterfly out of 2 - 10" rounds. Once carved, about half of the round is left, leaving the serving equivalent of 1-10" round.
I was just told today (for Sat. cake) that instead of 20 guests, to plan for 35 - 40.
So, according to the Wilton chart, a 2 layer 10" round should serve 28 ppl, which I fear will not be enough if I do a 2 layer butterfly. If I made a sheet cake to put under the butterfly using 2 - 9 x 13 pans, making a 13 x 18 essentially, 1 layer should serve 36, and the 1 layer butterfly would serve 14 for a grand total of 50. Does this sound about right? It sounds like a pain to shape a cardboard to put under the butterfly, though.
Another option is to make the 2 layer butterfly and supplement with cupcakes. Probably a cheaper option, huh? Going that route, do you plan on 1 cupcake/person, or would 2 make more sense? This is an adult party, btw.
What about taking two 12in hearts for the wings and trimming the points a little? You'd have 56 servings before trimming, and you could bake some cupcakes to place between the two to make the body of the butterfly.
How are you carving the round so that you only end up with half of it? The typical butterfly-from-round-cake design involves using half of the round for each pair (upper and lower) of wings, and the only part you carve away is a bit of a notch in the sides. (And you can then use those cutouts for the butterfly's head/body if you want.)
You can stack a single-layer butterfly on top of a single-layer sheet cake without a cardboard support. At least, I've done it without a problem, though I did not have to transport the cake.
One of these butterflies was made from a round (8" or 9", I don't remember which), and the other from two small hearts. The base is a 12x18 single layer.
The body of the pink butterfly is made from the side of one of the hearts used to make the purple butterfly -- I cut a curve out of the side in order to fit the two hearts next to each other with points together. The body of the purple butterfly is just frosting, piped in a ruffly mound using a "dessert decorator" (large tips).