I would suggest going with a combination of products to tame the noise, especially from a loud muffler. Alpha Damp + a noise barrier like Luxury Liner Pro would be the ticket.
But in terms of how much Alpha Damp, just a single pack will do it. 25% panel coverage and you'd be good.
You're looking at 1.5-2 sq. ft. per door, 2-3 sq. ft. on the roof, same for the rear wall, etc. etc.. so again, a single 20 sq. ft. pack should do it.