So much there is no room for fish to swim. That would be to much.
is there such a thing as to little?
Most definatly. If your using your live rock as a living filter than you should shoot for an absolute minimum of 1lb per gal
. For a 65 a happy amount in my mind would be 75-90lbs. Different types of live rock has different densities so 75lbs of fiji rock will take up a different volume of space than 75lbs of FL rock for example.
Place the rock how you want. Ideally you should leave room between the rock and the back glass and also you should situate the rock so its fairly stable and wont topple over yet at the same time not place it in such a manor like your building a rock wall. You want places where the fish can swim in and around the rock this will asure that water can flow thru the rocks. If the fish stay in the back behind the rock or not is more a product of the fish's tempermint than how the rocks are setup.