The best approach is to actually measure the areas you want to treat - total square inches / 240 = the number of tiles you will need.
Generally, 5-7 CLD Tiles on each exterior door skin, 2 more, cut into smaller pieces on the interior door skin and if you like, 2 more for the back side of the trim panel.
For a trunk. 10-12 on the floor, 3 around the sides of the spare tire well, 3 or 4 on each quarter panel, 2 or 3 on the trunk lid and 3 or 4 on the rear deck.
38 for a good job, 52 if you are going to pound on it.