Aztec Empire in 1519: c. 5 Million people.
Tenochtitlan (site now of Mexico City) Population: c. 200,000. By comparison: same size as Paris & Naples, the two largest European cities c. 1500 (roughly equivalent to contemporary Boise).
Hernando Cortez Forces: c. 500 soldiers, c. 200-300 Cuban and African slaves to portage, cook etc., 16 Spanish horses, 10 cannon, smaller cannon, crossbows, pistols, war-dogs etc.
Siege of Tenochtitlan casualties: c. 200,000 lives.
Cortez recovers, receives reinforcements, builds a small "navy"