My attention was originally brought to this by the book Lies My Teacher Told Me - I listened to an audiobook so sadly I can't really search it for the relevant passage without investing a bunch of time. Instead, here's the relevant Wikipedia article which covers it. "The population of Indigenous Americans is estimated to have decreased from approximately 145 million to around 7-15 million between the late 15th and late 17th centuries, representing a decline of around 90-95%."