Well, they can certainly estimate how many people could live off a particular parcel of land - taking into consideration that they were migratory as well (following the seasons). I suspect the warmer climates accommodated larger populations (where gathering was much easier); while the Inuits/Eskimos would be at the other extreme.
One of the civilizations (possibly in Mexico - Olmec?) actually disappeared before any whites ever arrived; the myth of the Eden-esque paradise destroyed by whites is dying.
Central Mexico was very fertile farm land back then and could have supported a large population. A lake once existed where Mexico City is today, and many Indian kingdoms existed along its edges and irrigated from it. They even had floating gardens on the lake. They were a smart and sophisticated people, yet, they let the barbarity of the Aztecs overwhelm them. Then came Cortes!