As I just removed the 4 carpets and washed them à la Murray, I can tell you that the rear ones have 3 and the front ones 2 (to avoid the carpet moving forward all the time.
What I want to do now, is to remove the original foam and heavy rubber/plastic under the carpets and install Dynamat. A pity perhaps to remove the original stuff, but Dynamat is surely a lot better. In the rear, there is the remains of some thin foamy stuff which I will scrape off and stick the Dynamat instead.
Most important, noisewise, is probably the front on the front footwells, where it goes almost vertical plus the prop shaft tunnel, but this I won´t remove just yet.....