I agree with silverado, I personally say go for as much rock as you like the look of, but at least have some as a good base.
I think yours looks like a good minimalistic amount. Depending on what livestock you add you may want to add more, but you can also add more later.
Some critters require more rock for hiding and eating, and if you want more than just a little coral you'll want more rock. Some corals are aggressive towards other corals, so gotta give them space.