I'm aware there were some graves under the western cemetery and quite a few in the cliff faces. I've never seen an estimate of the number of graves and their concentration. Even digging up all the cemeteries that exist at Giza might not unearth more tombs underneath. Just as I can't see through the pyramids i can't see through the mastabas either.
You might be interested in the geology and geography of this entire region. It is absolutely fascinating and unique on many levels. There was a time only 7000 years ago that virtually the entire delta would be inundated every year during the flood that later Egyptians called "high Nile". Crossing the delta under these conditions would be impossible by foot and dangerous by boat. Most travelers had little choice but to cross at Giza where the current was tame, the water deep, and the river narrow.
It's difficult to find much data because Egypt was under the Soviet Sphere until 1990. Much of the information is tough to find in English. Much of the most important information about Giza dates back to 20,000 years ago. In fact, the entire last 80,000,000 years has been important to understanding the pyramids and why they exist. Egyptologists have a very narrow focus. Without seeing the big picture it can be hard to tell what you're looking at. Sometimes you see only trees but it's really a forest.