Almost all of the New World's population was wiped out by Eurasian disease in OTL. Europeans rapidly colonized most of both America's and gained majorities. None the less the industrial revolution dramatically reduced TFR and changed global culture, and both America's to this day maintain a very low population density even with some of the world's premier farmland.
So, had European colonization began a few hundred years sooner, or if the industrial revolution was delayed a couple hundred years, how many people might fit in the America's? If the industrial revolution was even stunted 100 years could we see the current US borders with a China type population? Could the Southern Cone hold a population the size of Europe?