Not sure how many mixes you need. Do the pan instructions not say? An average mix makes between 41/2 to 5 cups of batter, depending on brand and whether you've doctored the mix. Most pans tell you how many cups to use. If not, fill the pan, by cup, to wherever you want the batter to go (1/2 or 2/3 full), count the number of cups of water you've put in, and that's the number of cups of batter you need. If it's 6 cups, for instance, you would need to make two mixes, using just over half the total amount of batter.
As for baking tips, you should probably use at least two flower nails in the pan itself, greased and inverted, to help distribute heat in a cake that size. I'd also use the baking strips around the outside of the pan to keep teh sides from baking way faster than the centre.
I agree with Jeanne G....I always use 3 mixes for a nice, full 2" high layer! The pan will not hold all of the batter....I fill it 2/3 full and make cupcakes with the rest! I tried it with only 2 mixes and the cake would not rise all the way flush with the top of the pan....the corners were shorter, so when I leveled it, I ended up with a really short cake!
I agree with Jeanne G and cami5271. Filling the 11 x 15 pan 2/3 full from the three mixes makes a really nice high cake. Using the flower nail is a must for making sure the cake bakes completely. I used the extra batter for a small 6" cake... one my two grandkids finished off rather quickly!