I use 2 boxes for a 9x13
2 1/2 for 11x15
3 for 12x18
They are nice and full
Check the articles section for the answer:
http://www.cakecentral.com/article-topic-4.html
I only use 1 box for a 9x13x2in. How tall is yours Lisa? I may have to add another mix and see if I get better results!
Oh, so that would be abt 4-5 cups for mine and abt 8-10 cups for Lisa's. I almost forgot to add that info! It depends on the mix you use how many cups of batter you get.
Here are all four Wilton cake preparation and serving charts:
(Shows how much batter needed by pan size, how long to bake at what temp., and more. Also give servings per pan by size.)
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
FYI: the sheet cakes are in the 2/3" party charts. 
A 13x9x2 takes 7 cups.
The WASC cake expanded flavors recipe:
(Using DH white cake mix makes 14 cups - a half recipe is perfect for your pan.)
http://tinyurl.com/2cu8s4
HTH
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