Need about 6 cups of rendered pork fat--how much solid backfat do I need to start with?
You'll need about 3 pounds. See here for a great step-by-step: http://homesicktexan.blogspot.com/2008/05/how-to-render-lard.html
I recently rendered 2.3 lbs and got about 2.5 cups. Also, depending on what you are using it for note that leaf lard is better for pastries (i.e. mild taste) than backfat.
I meant 4.5 cups.