Jturk, go to this site and download the free spreadsheet. It is a very nice BBQ catering planner which will figure your requirements for you.
If you don't have Excel to run the sheet, download openoffice software.
New Braunfels Black Diamond
Backwoods Patio Smoker
ET-73 Maverick Wireless Thermometer
Déjà Moo - The feeling you've heard that bull before.