In his book The Other Slavery, historian Andrés Reséndez gives an overall range of between 2.5 - 5 million Indigenous people enslaved by various colonial powers prior to the 20th century. He gives an extensively footnoted table in one of the appendices in his book, breaking down his estimates region by region in 50-year intervals. He also notes in an interview that he tried to be as conservative as possible in making his estimate, but due to the scarcity of sources his range is not very precise. I was wondering what the current consensus is, if any, on his numbers.
As far as I can tell, when Reséndez was working outside his direct area of geographic and temporal expertise, he cited the best work of other scholars in the field. Because the indigenous slave trade, unlike African slavery, mostly operated in the black, or at best quasi-legal/unpaid labor, market we lack the hard data for sales, purchases, and taxes associated with the ownership of indigenous slaves. Almost every historian of indigenous slavery works with incomplete data, and will err on the side of conservative estimates to reflect this absence. I haven't encountered any scholars who strongly disagree with Reséndez, mostly because he did a great job of drawing from the right experts to compile data for The Other Slavery.