idgy - it depends also on the type of rock, as to how much room it will take up. Some rock is more dense than others, so one piece might weigh more compared to a similar sized rock that is more porous. Something to keep in mind - when aquascaping, try and allow for good flow and avoid stagnant spots where detritus might collect. It will make a great spot for algae. A solid wall of rock may not be a good choice if it inhibits water flow. Also, don't be afraid to break up pieces to create shapes you are looking for, as plack indicated by drilling holes he will be able to put rocks together in ways that might not be done by only stacking rocks.
Depending on the fish you will keep, they may enjoy a lot of live rock - it will provide them with hiding spots, territory, a live food source, etc.