The Templo Mayor was the central pyramid in Tenochtitlan.
The pyramid was constructed with brick and an outer layer of lime plaster. It had two shrines on the top, one dedicated to the Mexica god of war Huitzilopochtli, and the other to the Central Mexican rain deity Tlaloc.
The swampy environment of the island of Tenochtitlan necessitated the construction and fixation of thousands of structural pylons to support large structures. In addition, the Templo Mayor was constructed with porous, lightweight volcanic rock. These factors combined to ensure that the sinking of temples into the swampy landscape was minimized. The original temple was quite small, but it was built over six times throughout the history of Tenochtitlan. As the affluence of the Mexica grew, so too did the temple. The final iteration of the Templo Mayor was approximately 100 by 80 meters at its base and about 60 meters (200 feet) tall.
Human sacrifice took place at both shrines on top of the pyramid, with anywhere from several hundred to several thousand people being sacrificed each year. The exact numbers are difficult to determine and the estimates by the Spanish conquistadors were massively inflated. That being said, up to 20,000 prisoners of war were sacrificed over a four day period during a religious celebration in 1487. The Spanish estimated 80,400 sacrifices, but this is undoubtedly exaggerated, simply because it is logistically infeasible. The Aztecs themselves estimated 20,000, but this too could be an exaggeration because it served in the best interest of the imperial government to propagate expanded sacrifice projections.
In any case, the number of skulls present on the Hueyi Tzompantli (great skull rack) suggests that at least tens of thousands were sacrificed over the course of Tenochtitlan’s history.