Well as much or as little as you like and/or can afford. Just make sure you have sufficient flow around the rock pile/wall you build. I would also suggest making lots of caves and overhangs so the fish have places to hide and swim in and out of. Have a look around this site at people's aquascaping and you'll get the idea. I cheated a bit and stacked mine in one place against my overflow which will restrict flow around it a bit but I don't think I'll have a problem.
I would suggest getting more dry rock than live rock and to seed the dry rock with a few lbs of live rock. It will save you a lot of money!!! Dry rock once seeded become live rock