Josh-
Most packets SHOULD (but for some reason many don’t) have directions on them- stating how large of a batch the yeast is good for.
I always add 4 grams of dried yeast per gallon of mash. If you are doing 20 gallons I would add 80 grams or 2.82 ounces of dried yeast.
I am sure you already know- I’m not a huge fan or turbo yeast unless it is being used on a 100% sugar mash. I feel it leaves some not so desirable flavors behind. If you have not tried other yeats I’d give them a try. Brewers yeast, distillers yeast, ale yeast, bread yeast, etc.