Yeah, both ends of that range are way too high for North America, but about right for America as a whole.

You could imagine a civilization that fed 100 million or even 600 million people on food raised on North American arable land, and in fact such a civilization actually exists today, feeding about 600 million people; North America is a net exporter of food, and its population is about 592 million. Currently that's being done with industrial agriculture mining phosphate rock and fixing most of its nitrogen with the Haber–Bosch process because that is far less labor-intensive than the alternatives, but some varieties of biointensive cultivation are actually almost competitive with current industrial agriculture when you measure by yield per acre instead of yield per hour of work.

However, biointensive agriculture didn't exist 500 years ago any more than the Haber–Bosch process did, there's no archæological evidence there for such dense populations, and as far as I know no mainstream archæologist suggests a pre-Columbian population of North America anywhere close to 100 million.