It is hard to answer this question based on mixes, because different brands and different flavors inside of the same brands make different amounts of icing.
For a 2in tall 13x9 you need about 7 cups of batter. If you are using a white cake mix from say BC, you would need almost two cake mixes since I find that the white mix makes about 4ish cups of batter. If you are doing a chocolate cake, those tend to make a little more and rise higher so I'd say probably closer to one and a half mix.
If you use the DH Golden butter it actually makes 6 cups of batter (last time I used it) so then you would only need a little more than one mix.
The best thing to do is measure out sometime how much the different cake mixes you use make, and make a note of that. If you scratch bake then do the same for the recipes you use.
Also have you heard of the extender recipes? They bulk up a cake mix, so when you have a case where you just need a cup or so extra they are nice.
This is pretty much the one I use. http://cakecentral.com/recipes/1977/cake-mix-extender