If you look at the Aztec and Mayan civilizations, you’ll see larger numbers. But north of Mexico, the numbers were probably never more than five million. The historians looked at the absence of cities and the nomadic life of most Indians and calculated that it would have been very difficult to build up large numbers of people. We must be wary of work done by leftist academicians who look to inflate numbers for political purposes.
The first Indians that the English and Dutch encountered on the east coast (1600s) had sizable populations and stayed in an established range for the most part but did not have the structure found around Mexico City and south.
All these populations were cut down mercilessly by having never been exposed to European disease. Africans had been exposed, thus made better slaves for the Spanish and other Europeans.
Mann mentions “over-counters” and “under-counters” who have political issues they want to put forward. I'd recommend both his books.