We do not know how many there were,
there were numerous cities in the amazon - one was estimated to be around 360,000 many others were about 10,000.
30,000,000 is the estimate of those who died of disease in SAm and over 110,000,000 in NAm
After the mass die off there were only scattered handfuls in SAm and in NAm there were only a few bands and tribes left by the time of Lewis & Clark - most having died unseen by Europeans.
Those numbers are close but it is well documented if you care to do the work. America Before has many of those numbers and sources in the footnotes
There were certainly higher population levels in the Amazon than previously thought.
Tens of millions is likely greatly overestimating the numbers.
We do *not* know when they died off or why.
I have seen articles that claimed they died off long before the Europeans entered the scene.
The great plains of North America were nearly uninhabited when Coronado crossed them in 1540-1542. There were much higher populations along both coasts.
110 million seems a very high number to me. They did not have sophisticated agriculture, draft animals, or metal working.
Early Spanish ship-wrecked sailors drifted from village to village, starving.
The premise of very high populations seems to rely on a huge, extremely quick die-off of about 80% of the population. Hard to prove or disprove.