Fist of you need to know the ABV that the mash is. then you need to know the efficiency of your still. If your Mash is 10% ABV and your Still is 90% efficient (meaning how much of the total ABV is collected there is no still that is 100%) you should collect 1.5 cups of 180ish proof. per gallon of mash.
My setup 4.5gallons mash at 14%ABV Threw homemade Reflux Still (4 stage) I collect over 7cups of >93%ABV(drink-in) and just over 2cups 70%ABV for putting into next batch before distillation. I also use this portion for cleaning and fire starting ext.
Hope that helps.
Also here is a good website for every thing that you will need for starting out http://homedistiller.org/ http://www.milehidistilling.com/
Jun 25, 2010
Making Moonshine and How Many Gallons per Mash? by: Countryfarm Lifestyles
Unfortunately, when you make moonshine, the proportion of moonshine is far less than the amount of mash you start off with.
For example if you have 4 lbs of corn and 5 gallons of water, along with everything else (malt, sugar and yeast) you will only get about one and a half (1 1/2) gallons of moonshine.
So if you are only going to start off with 2 gallons of mash you won't have much moonshine at all!