how many servings should I be able to get from a single layer 9x13 cake, with no filling?
What about with filling and two layers?
I can't seem to find a chart anywhere.
I use the numbers off this chart. It is for two layer cakes. So a double layer 13x9 would serve about 50 1x2 in pieces. For a single layer you are looking at about 25 (probably 24?) 2x2 in pieces.
Here's a link to metria's great cake calculator:
It lets you calculate servings (and a lot of other things) either by volume or by the Wilton cake charts. It's been a lifesaver for me. I've put it in my internet favorites, because I refer to it so often.