Encyclopedia of the Incas

Inca conscripts were male subjects aged 25–50 performing their labor service,
who had little specialized military training. Although the Incas came to prefer
certain ethnic groups for garrisons or for the emperor’s guard, such as the Cañari
and Chachapoya, they never developed a professional army, relying instead on
forces that could be quickly mustered and disbanded. At the emperor’s call to
arms from the ushnu in Cuzco’s main plaza—chronicler Pedro de Cieza de
León called it the “stone of war”—the word passed down through the provincial
governors and Native lords to call up men through the decimal hierarchy. By the
contact period, Inca armies numbered in the tens to hundreds of thousands.
Atahualpa reportedly had 40,000–80,000 soldiers at Cajamarca. In 1533, 35,000
troops were stationed in the provincial center of Hatun Jauja, according to
accounts of quipucamayocs (see Quipus). Manco Inca mustered at least 100,000
troops with 80,000 auxiliaries at the siege of Cuzco. Such numbers speak to the
unprecedented scale and efficiency of Inca administration.