According to Historian William Denevan, " 54 million people inhabited the Americas in 1492, including 3.8 million above the Rio Grande." Source. That means that only around 7% of the Native peoples lived in what is now The United States and Canada. Why such the disparity? A Rainfall map will show that the Eastern half of the US is extremely fertile and, presumably, easily suited to human settlement. Why were the peoples of that region so sparse compared to those in the South?
I know this isn't going to answer your question but I thought I would bring up a point. That 54 million number is not set in stone and is debated by many historians. Charles C Mann in his book 1491 states that the population could easily have been over 200 million at certain points before 1492. And during 1492 anywhere from 100-200 million. There is no sure fire way to prove who is right but Archeological evidence has been upping the estimates since the turn of the 20th century.