How many cups of cabbage is a 12 oz bag?
Hard to say, since how tightly it is packed makes a difference. Hang a coat hanger from a kitchen cabinet. Using a clothespin, on one end hang a 12 oz bag of anything you have, or something like a 12 oz can or two small cans of tomato paste in a ziploc bag, then hang another ziploc with shredded cabbage on the other end and add/subtract till they look level. You'll be quite close to 12 oz.