I am using a Duncan hines cake mix. I am making a double layer 10 inch cake with only one pan. So how much cake batter should I use?
Each box of Duncan Hines cake mix yields approximately 5 cups. Per the Wilton chart you need 6 cups per layer for a 10"round cake. So you will need 3 boxes of cake mix. With the extra batter you could make some cupcakes and throw them in the freezer to eat later.
My vanilla cake makes about the same batter as a box mix and I use one batch per 10" pan, but I also make 3 layers of cake. If you want 2 layers, follow @Freckles0829 advice.
I would use one box per pan if they are 2" tall pans.