I searched around for this and was surprised how little data I could find. I want to know the population of all tribes that Genghis Khan had under his control when he became the Great Khan (1206). Hasn't there been some study and some estimate by any historians?
According to the Atlas of World Population History (by McEvedy and Jones), it was about 800,000.
... the demographic base was of the same order of magnitude about 800,000 when in the 13th century Genghis Khan set out from Mongolia to conquer the world, and it was still in the same band in fact slightly lower, about 600,000 when the Chinese established control over the country in the 18th century.
Here's what they wrote about their sources:
A. K. Validi (quoted by • Russell, p. 87) suggests a figure of 0.5m for Mongolia in the 6th 9th centuries AD. This is no more than an informed guess, for only one figure of use survives from the pre-modern period: the size of Genghis Khan's army. This was established at 129,000 men, with H. D. Martin, The Rise of Chingis Khan and His Conquest of North China (1950). p. 14. considers compatible with a Mongolian population of around 0.75m. C. R. Bawden (The Modern History of Mongolia (1968)) quotes a mid-19th-century Russian estimate of ‘not much over 0.5m,‘ and G. S. Murphy (Soviet Mongolia (1966)) one made in 1918 of about 0.7m. The first census was taken in 1956, the second in 1969.
This implies to be there's a great deal of guesswork in this number based on the only historically reported fact available (a report of the size of his army). So it should probably be taken as no more than a rough order of magnitude estimate.