Your population estimate cited for Mexico is similar to those used to portray the US as chockfull of Indians before evil whites came and eradicated them. In fact, their primitive cultures and technologies prevented them from ever sustaining large populations close together. Some historians maintain that the plains Indians weren’t even there when whites arrived, but rather are descended from tribes on the East Coast that were pushed westward.
Indians in Canada don’t gripe because rather than move them the Europeans could usually leave them where they were; they weren’t large populations and there was plenty of land.
There is plenty of room for those estimates to be off. A few cultures did exist on the Great Plains before 1700, but most did not move onto the Plains until after the arrival of horses. The Pawnee and Wichita did inhabit river valleys along the Platte and Arkansas rivers beginning around 1300 or so, but they were exceptions. The most common estimate for the number of Indians north of Mexico when Columbus arrived is 8 million. Truth is we can’t do much more than guess.