Hi rural chick.
I am thinking that 2 cake mixes should be fine in that wonderful pan, and bring it up to about 5/8 inches below the top of it when baking a cake....and I'll bet it will accept 3 cake mixes, bringing it up above the top but it should puff up above the top and be great. (Leave the oven closed for the duration!)
I would love to have that pan to make my Yorkshire pudding in! I always make a big batch of it and I never use the muffin tins. Your pan is what I need!
Congrat's on your purchase!