You need a tape measure, a piece of paper, and a pencil. Decide what you are going to cover (floor, doors, etc) then take some rough measurements of these areas. Add up the area, and that's approx how much you need. You'll get alot more accurate idea of your situation than a bunch of guesses from us.
The rule of deadening is, some is good, more is better //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
Can't hurt...if it was me, I'd do something like two layers on the roof, two layers on the floorboards (could use asphalt based deadener to save money here), and as many as needed on the doors.