According to Jared Diamond's Guns, Germs and Steel (p. 211), the Mississippi Valley used to contain populous Indian societies:
When we in the United States think of the most populous New World societies existing in 1492, only those of the Aztecs and the Incas tend to come to our minds. We forget that North America also supported populous Indian societies in the most logical place, the Mississippi Valley, which contains some of our best farmland today.
In the following paragraphs he also claims that archaeological evidence suggests that North America in total had about 20 million population before Colombus' arrival. What was the estimated population in the Mississippi Valley before Colombus' arrival, and how large area did this society cover? (I want to know this in order to get an idea of its population density).
And how did historians come to conclude the number?