Depends on how much is going to be MDF and how much is going to be fiberglass. A gallon should do it.
I did a spare tire enclosure a few months ago that had 2 gallons in it. It was a little over 2 cubic feet.
Really also depends on if you are using a lot of fleece. Fleece will soak up more resin than mat will. So if you are using a lot of fleece to get a shape plan on using more resin.