Its about 1.5 lbs of LR per gallon of water. And that varies slightly depending on the type of live rock, but thats probably your best guideline to follow. Live rock acts as a Bio filter.
I suggest ya start with a FOWLR (Fish Only with Live Rock) system, as its a bit easier to set up and get running, than ya can add equipment and corals in the future once it gets established. Now, im SURE that somebody is gonna disagree with me, but in MY EXPERIENCE, a FOWLR is a lot easier to successfully set up.
Also, i suggest ya look at a couple charts chart that shows ya what fish are compatible with other fish if you plan on having more than just clowns. Heres one that looks fairly decent. But you might want to find another one to compare them.
http://www.peteducation.com/article....articleid=2605