Ha ha. At least 8 is reasonable!
"By one series of estimates, the population before the time of the conquest is estimated at 19 million" -- http://en.wikipedia.org/wiki/Aztec
"Población
• 1521 est. 20,000,000" -- http://es.wikipedia.org/wiki/Imperio_azteca
"Consistía aquella armada en 11 naves, con 518 infantes, 16 jinetes, 13 arcabuceros, 32 ballesteros, 110 marineros y unos 200 indios y negros como auxiliares de tropa. Llevaban 32 caballos, 10 cañones de bronce y 4 falconetes." -- http://es.wikipedia.org/wiki/Hern%C3%A1n_Cort%C3%A9s
"11 ships, 500 men, 13 horses and a small number of cannons" -- http://en.wikipedia.org/wiki/Hern%C3%A1n_Cort%C3%A9s
So, between strength 21,372.328458942632170978627671541 and 40,000 for a Spanish army. That sounds good.