Given that this is how American Buffalos went from 30 millions to only 300 in 50 years,it's over 10 million that's for sure.

Huge herds of buffalo were actually something of an anomaly and they didn't use to be so numerous until just few centuries earlier. Early European explorers of the areas where buffalo herds took place don't even mention them, even though giant herd of giant animals would surely been notable. Apparently Indians kept the population in check, since who wants countless giant animals stomping on their fields?

However, after Columbus multiple waves of foreign diseases depopulated many areas, and so by the time Europeans arrived to settle, buffalo numbers had grown enormously in the absence of hunters.