60,000,000.

Muybridge, Animal Locomotion, 1887

According to naturalist Ernest Thompson Seton, North America at the time of Columbus was home to sixty million bison, thirty to forty million pronghorns, ten million elk, ten million mule deer, and as many as two million mountain sheep. Sixty million bison!  The imagination shrinks from imagining it.  Bison can run for hours at thirty miles an hour and use their massive horned skulls like battering rams.  Mature animals weigh up to a ton. Sixty million of them would have been more than sixty billion pounds of grouchy, fast-moving mammal pounding the plains.

Charles C. Mann, from 1491: New Revelations of The Americas Before Columbus