Four wheels, two empires, and one conqueror.
As far as wheels go in Central and South America, they didn't catch on mainly due to the terrain. A good portion of the Aztec, Inca, and Mayan civilizations were up high in mountainous terrain - and they didn't have hard metals either. IIRC, they didn't even have copperworking down to an art, which is why they used obsidian-edged clubs. This made wheels quite impractical, and instead they used pack dogs and slaves to carry goods.
There were actually three empires, but only two when the Spaniards came calling - the Mayan empire had collapsed inward before they got there. (Or was it the Inca? I don't remember.)
The Aztecs were actually winning the fight - despite firearms, real armor, and superior weaponry, the sheer numbers of the Aztecs were overwhelming the Spanish forces. Cortez made a push for the leader of the Aztecs (I don't remember his name) and managed to kill him. The thing is, the Aztecs revered him as a living God. If you can kill God, who wants to fight you?
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche