How much mud do you need to tape 6 4x8 sheets of drywall?

Assuming the following:

2" wide tape

4 coats of compound: 1 base (tape adhesion) layer and 3 finish layers with each roughly the same amount or mud but spread over a progressively wider area

For area calculation purposes each layer has a center thickness of about 1/32" tapering to zero over 2" (1" of tape plus 1" feather per sheet of gyp) or the equivalent spread over a larger area.

Doing the math (might want to check it!):

You will need 6.5 to 7 gallons of compound.

If you have three layers total:

0.703 cu. ft. with (.133 cu. ft. = 1 US gallon) = 5.25 Gallons. Mathematically, you would round down, but tectonically, it's probably better to round up to six gallons...You know, in case someone trips over the bucket....or screws up the math! Good Luck.