The reef tank I kept a couple years ago utilized only live rock and a skimmer for filtration. I believe it is called the Berlin method.
I did not use a sump, because my tank was small, but others have incorporated a sump into their setup, usually filled with live rock.
Works great. I kept a tank using this method for over 4 years and it worked fine. I don't know if there is hard and fast rule as to how much LR to use, but my tank was 37 tall, and I used about 50-55 lbs of LR in the tank, without a sump. If you have a large sump, you can place more of the rock in the sump, and free up some space in the tank.