1 layer 9 x 13: 1 box (won't bake up very high), you might want to mix up two so you can fill the pan a little more and make some cupcakes
11 x 15: two boxes (entire batch of batter)
12 x 18: 3 boxes: may have a little batter left over
Hi and Congrats on your 1st post, Lemasmom. 
How many boxes of cake mix does it take for one layer sheet cake?
Since sheets cakes come in numerous sizes you need to be more specific. If this cake is for a customer, it's advisable to let them know your usual serving size is, and then ask how many servings they want. Then bake a cake that makes that amount of servings....
They could be thinking a sheet cake for 12 servings and you're making a cake that serves 90...
Everything you need to make, decorate and assemble tiered/stacked/layer/sheet cakes:
http://www.cakecentral.com/cake-decorating-ftopict-605188-html
There are links to Wilton's cake preparation charts which give sheet cake sizes and batter requirements by pan size. Obviously, if your sheet cake is only a single layer, you'll need to half the servings yield given. And a 2x2x2 is equal to a 1x2x4 (wedding) serving while a 3x2x2 is equal to a 1-1/2x2x4 (party) serving.
The above link also has several versions of the WASC doctored cake mix recipe which yields a tad over 14 cups of batter (when made with 2 DH white cake mixes).
HTH
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