How many cups of batter does a cake mix make?
A 9&quot; round pan that is 1 1/2&quot; deep takes 6 cups of batter, while one that is 2&quot; deep takes 8.  That's for totally full to the brim.  This source says about the amount of batter from a box:

An average 2-layer cake mix yields 4 to 5 1/2 cups of batter.

and should have a bunch of helpful guides to using batter for non-standard pans.