Just stopping in, and saw this.

It all depends on how much surface area you want to cover, and what material you will be using. Some papers are more porous than others and will require more "glue". Fabrics soak up even more the the stuff. Technique makes a difference as well. Sometimes you sort of 'soak' the pieces in the mod-podge then apply to the piece, smoothing it out as you go. Others you spread a layer of the mod-podge, smooth a piece over it, then overpaint it with more. Many times you will want to let the piece dry, then give it another full coat of the podge to finish it, and even then glaze it again using something else (such as water based glaze).

Some brands give you an idea of coverage in square feet, but still it is hard to predict.