In my 20 gallon FOWLR tank, I only have 7 pounds of live rock. I have three pieces stacked to make a little tunnel for fish to swim through, and feather dusters are living on it. It is only located on the left side of the tank, and on the right, I have a large piece of dead coral as decoration.
In my 27 gallon FOWLR tank, I have 25 pounds of live rock. They are stacked around the sides and back of the tank, and they are only a little bit less than one third of the tank's total height. There are plenty of places for fish to swim and hide in it, and all the LR helped cycle the tank in only 2 weeks. (The 20 gallon tank took 5 weeks!)
You can get as much LR as you want. It's your preference for FOWLR tanks.
I would also suggest buying the rock from SWF.com since they seem to be cheaper than most LFS.
I got most of my rock from here and am very VERY pleased with it.