The satellite view shows this off much better than Wikipedia's ground-level picture. It Really is just a long band of holes dug into the side of a mountain.
At 1m diameter and 75cm deep, so ~0.59m^3, I calculate that to fill the Albert Hall, which a search suggests "has been estimated at" ~100,000m^3 (feels low to me, but it's quoted in many places), it takes around 170,000 of these Peruvian holes.
https://www.google.com/maps/place/13%C2%B042'20.0%22S+75%C2%...