There's no one reliable figure. Estimates for the pre-Contact population of the Americas alone range from 15 million to 120 million: by 1650 fewer than 10 million remained. So the population loss there (overwhelmingly the result of unfamiliar diseases) could be a few million (probably too low) or 100 million (probably too high). Similar uncertainties plague estimates of Australia's Aboriginal population before white settlement: 300,000 or a million? By the 1920s there were fewer than 75,000. Then there's the question of populations displaced or assimilated earlier of whose fate we know still less: where did Bronze Age European peoples go? And excess deaths don't necessarily end with depopulation: even a recovering population may live in circumstances that shorten life; conversely it may later share to some extent in a wider improvement in mortality.

The best answer I can give is millions, possibly tens of millions. There is no simple answer, and any one figure should be questioned.