Yes, you are right. Before Columbus arrived, there were about 100 million native American Indians. Today, there are about 5 million
I have read as few as one million in what is now the United States. There were far larger Amerind populations south of here. Explorers and settlers did not run into huge population concentrations as was the case in Mexico. There were some fairly sizeable Indian towns in the Ohio Valley, but there were also enormous areas where nobody lived full time. The Ohio Valley tribes, for example, cleared Kentucky for a private hunting ground and wouldn’t let anybody settle there. In my own Ozark region, the Osage moved down from (probably) Minnesota and settled in the Missouri Valley. They cleared the Ozarks of all other Indians, again, for a private hunting reserve. The Indians who built Cahokia, the largest settlement along the Mississippi, were gone by the time settlers arrived.
The Great Plains couldn’t support large populations because it was difficult to grow or kill food there until the horse allowed Indians to follow and chase down the buffalo. But even after the population explosion caused by Indians’ acquiring horses, the plains tribes were never very large. Before whites arrived, the Comanche drove the Apache from the southern plains, but probably never numbered more than 45,000.
When settlers arrived, they entered largely unpopulated or underpopulated areas.
Las Casas wrote about Mexico. was probably right in saying many Indians in Mexico were slaughtered. But a lot of that was by other Indians. Cortez’ army was largely an army of Tlaxcalans, a tribe that hated the Aztecs. After the Spanish victory, many of them became “Spanish” grandees themselves.