So lets start at the beginning.. First, there is no limit to how much rock you can have in a tank. The amount of rock depends entirely on the tank owner. As long as there is adequate flow around the rock to prevent dead spots for unhealthy bacterial growth, the sky is the limit!
In general, it is accepted that approximately 1 pound of rock per 1 gallon of tank space available is a fair goal. IE: if you have a 120G capacity tank, 120 or so pounds of rock. This is a guideline, not a rule.
More importantly, Are you creating the most natural habitat for the stock you intend? A pile of 120 pounds of rock in the middle of the tank, may meet the guideline, but will it create a place for your fish to live and thrive?
Fish need a home. Caves and pass-throughs are vital. Can your flow reach all of these areas? If you intend a reef, are you providing space for growth? Enough area under the lights for corals to survive?
Lastly, and least important for the tank, but highly valued to the observer, is your scape visually appealing?
My personal scape, I tried my best to address all of these issues. "Open and airy" with space for light to reach most areas, yet places for fish to make their home, and a visually appealing (at least to my eye) design.