The exact number of Aztecs killed by the Spanish during the conquest of the Aztec Empire is difficult to determine. However, historians estimate that the population of the Aztec Empire was around 20 million before the Spanish arrived. By the end of the conquest, the population had drastically declined to about 2 million.
This massive decline was not solely due to warfare. Diseases brought by the Spanish, such as smallpox, played a significant role. Smallpox alone is believed to have killed millions of Aztecs, as they had no immunity to the disease. Additionally, the brutal warfare, forced labor, and harsh conditions imposed by the Spanish also contributed to the high death toll.
In summary, while the exact number is unknown, it is clear that millions of Aztecs died as a result of the Spanish conquest, with disease being the primary cause of death.