It is incredible. The reason they could kill so many is that they were at it for several hundred years and attacked different regions. So the population had some time to replenish before the next series of genocides started. Individual campaigns had death tolls in the 10-50 million range. There were 6 major campaigns. Several had multiple decades of genocide.

And of course, death tolls are a guessing game, as are population figures. So yes there are legitimate arguments about these figures.

http://en.wikipedia.org/wiki/Mongol_invasions_of_India