Just FYI, this figure that gets used comes from the high end of a range that is estimated from 20 million to 100 million for a population of North America around 1500. The estimate for Europe is around 50 million to 60 million.
https://www.quora.com/Is-the-claim-that-100-000-000-native-A...
Entire major temple complexes overrun by South American jungle have only been found because of modern deep scanning radar, so I feel it's pretty likely that the number is on the higher end compared to estimates from even 20 years ago.