Do you really mean gallons? Anyway, to work out the pitch-rate for your brew we need to know the gravity. Then to work out how to breed enough yeast cells to achieve the pitch-rate we need to know how old your yeast pack is because a live pack of liquid yeast starts dying the moment it's packaged. Then it's off to the starter calculator to work out how big your starter needs to be.So how many slap packs would I need to ferment a 35 gallon batch of wort, and would I need to make up a starter? If so, remind me how to do it - it must be over 50 years ago when I last did that!