Oh, sure it was. The high number that I've ever heard for aboriginals was 6 million, which I think is a crock and irredentist propaganda.
But even if it were true it's a tiny number of people for such an enormous area.
Lots of photographs of the American West exist which predate most of the significant settlement. The land is utterly empty. As a lot of it still is. Oddly, I have never seen a picture of Aztec airports, Ute skyscrapers, or Navajo nuclear power plants. Certainly weren't in any pix I ever saw. Guess they took them all down and hid them, huh?
It was European civilization which brought the institutions and technology to make the land support far higher numbers than what was here. You can argue all you want about how Asia had such culture (only to a subsistence level); or how it might have evolved eventually here (they had about 60,000 years here....about the same as Europe. So when would it have happened?); or...or...whatever other simpering, We're-just-as-good-as-ju-damn-gringos argument you can think of....but the truth is, the Anglo-Saxon settlers brought their culture and their genetics here and that's all she wrote. The rest is undeniable.
So, when the "settlers" arrived even Mexico had up to 50,000,000 people. The Iriquois nation could command the resources of possibly 4 to 5 million. There were millions more all over the place.
Who told you the continent was empty?
Americans invented and developed those features of the modern world. Americans built the steel skyscrapers.
What Europeans and East Asians do is a pale imitation of the real thing.