The exact number of Aztecs killed by Hernán Cortés and his forces during the conquest of the Aztec Empire is difficult to determine. However, historians estimate that the death toll was in the hundreds of thousands. This includes both direct casualties from battles and the indirect effects of warfare, such as disease and famine.
One of the most devastating factors was the introduction of smallpox by the Spanish, which decimated the Aztec population. The disease spread rapidly among the native population, who had no immunity to it. This, combined with the brutal warfare and the destruction of their cities, led to a significant decline in the Aztec population.
In summary, while the exact number is unknown, it is clear that the conquest led by Cortés resulted in a massive loss of life among the Aztec people.