Hmm, I just checked out a few online macaron tower tutorials and I found one that used 80 macarons for a 14-inch tower and another that used 100 macarons on an 18-inch tower. then this one on a 9-inch cone that used 30 macarons: http://thirstyfortea.com/2013/09/13/diy-french-macaron-tower/
So I think you probably need a 12-inch cone???????
Your best best is to figure out the total surface area of the cone and then do the math how many macarons will fit on there.
It will really depend on how big your macarons are! Good luck.