I use the cake mix extender (recipe on the site) and it makes a DH box mix yield about 7 cups.
According to Wilton, a 9 inch pan uses 5.5 cups of batter, but they always call for less than what fills a pan. So I would say 2 extended mixes would do the trick. If you don't want to use the extender, then I would say three boxes.
Stephanie
Here is a link from the homepage that tells you exactly what you need for batter/icing for the 2 inch pans. There is also one for 3 in pans if you ever need it, you can find the info on the home page. Here is the link and I hope it helps some, Jen
http://www.cakecentral.com/article14-Cake-Baking-Cutting--Serving-Guide---2-in-Deep-Pans.html
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