Some estimate before Columbus there were probably around 30 million in the US alone.
The estimate of the native population size varies greatly. Some estimate it to be closer to 200 million. We'll never know, of course, but it was substantial, and whatever it was, it was both accidentally and purposely decimated. The first Spanish explorers going up the Mississippi, and finding endless Indian farms and villages on both sides of the river, didn't know that the pigs that they were carrying with them for food carried diseases that would wipe out those populations. (The next group of explorers to follow that path, 13 years later, saw no signs of people, or at least no signs or large populations like earlier reported.) Later the U.S. Army knew damn well that they were handing out blankets infected with smallpox. California paid bounties on dead Indians until early last century. However you measure it though, none should have died, millions did.
The large buffalo herds encountered by white explorers were apparently a byproduct of the diseases introduced by the early European explorers, which killed so many people and hence reduced hunting pressure on the animals, allowing herd size to explode. The same apparently goes for the Passenger Pigeon, which early settlers said flocked literally in the millions. The population of those birds too grew incredibly once the hunting pressures were taken off by the many Indian deaths.
We Europeans really know how to f**k stuff up. But at least we are still upholding the tradition, weather and other-wise.
None of which will be mentioned in the christian version of history. Even the secular historians seldom mention these things.