A horde of people
Note: the answer is around 100. In other words, roughly 100 people, doing nothing for 12 hours a day but lifting stuff, would barely generate enough mechanical energy simply to get the pyramid blocks in place! When you think about the addition of friction, carving the blocks, feeding people, moving blocks along the ground, etc. you rapidly understand why building the great pyramid required the effort of tens of thousands of people over twenty years.