How many cake mixes does it take to make a 16" cakes 

Each mix will make about 4-1/2 cups of batter.  According to the Wilton chart:

http://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm   

It should take about 10 cups, or two boxes of mix to make two, 2" high x 16" round cake layers.   Personally, I'd use 3 boxes of mix and make cupcakes with the leftovers since I like my cakes to be a full 2" high after they have been baked and have completely cooled.  I use leftover batter for cupcakes which can be frozen and decorated later.

Actually, the chart Apti linked says that one 16" round layer which is 2" high will take 16 cups of batter.  That would be between 3 and 4 boxes of cake mix per layer.

16 inches is a big cake--100 wedding size servings for a 2 layer cake.

Also, if you haven't baked one before, make sure ahead of time that the 16" pan will fit in your oven.  It just *barely* fits in mine.

Magda_MI~~Thank you for catching my error!  Apologies, LeoAngel3, I was looking at the frosting, not the batter.   ooops..... 

Tell you what, here's a way to find out how many cups of water it takes to fill your 16x2 pan.    Take your 16" x 2" pan.    Start pouring (measured) cups of water into the pan.  See how many cups of water it takes to fill the pan half-way.  That is how many cups of batter you will need for each 2" high layer.   [Magda_MI, you better check this too, it's late and my eyes and brain are fuzzy...]

Looks reasonable to me.  I usually just go by the Wilton charts, though I find I sometimes need to add a little more than they say to get the full height for some of my larger pans.  Your mileage may vary.  And it doesn't help that the cake mixes have gotten smaller since I first figured out how many I needed for what size pan.