In twenty-five years, the Mongol army subjugated more lands and people than the Romans had conquered in four hundred years. Genghis Khan, together with his sons and grandsons, conquered the most densely populated civilization of the thirteenth century…Genghis Khan conquered more than twice as much as any other man in history…At its zenith, the empire covered between 11 and 12 million contiguous square miles, an area about the size of the African continent and considerably larger than North America…The majority of people today live in countries conquered by the Mongols; on the modern map Genghis Khan’s conquests include thirty countries with well over 3 billion people.
That’s from Jack Weatherford’s overstated but fascinating Genghis Khan and the Making of the Modern World.
By the way, the entire Mongol tribe was no more than one million people, with perhaps one hundred thousand warriors.
Today a Mongolian restaurant in London is offering free DNA testing. If you are verifiably a descendant of Genghis [what are you being tested against?], they will feed you a meal for free. The analysis takes two months, but supposedly you have a good chance of winning:
It is estimated that 17 million people worldwide, including the British Royal Family, Iranian Royalty, and the family of Dracula, are direct descendents of Genghis Khan.
Addendum: Read more here on the nature of the test.