This is a question that has fascinated many people for centuries, because we can see the outsized effect the Mongol Confederation had on Eurasia and we know that a relatively minuscule population compared to many of the states and empires it conquered… and also because the closer we look for answers to that question the more of a lacuna it seems to become. The tribes of the steppes were disunited, for the most part illiterate, and not concerned with keeping records of their numbers of migratory patterns. Accurate or even reasonably adequate estimations of the Eurasian steppe population is extremely spotty – pretty much to the point where it’s less “spotty” than just a “gaping void of information,” especially prior to the establishment and consolidation of the Mongol Empire ca. 1270, when the first attempt at an actual census was undertaken.. Nevertheless, we can attempt to cobble together some semblence of what the demographics of the eastern steppes might have been in the mid-12th century around the time of Temujin’s birth.

We might first look at what we can not just of the scattered Mongolic and Turkic tribes in the 12th century, but also their predecessor confederations, such as the Xiongnü, Göktürk, and Uighur Khanates in order to get a “baseline” for what other peoples – specifically the Chinese – observed about their numbers. A Chinese eunuch who had ventured into the Xiongnü Empire and then reported back his findings said the following, as written in the Book of Han:

All the multitudes of the Hsiung-nu nation would not amount to [the population of] one province in the Han empire. The strength of the Hsiung-nu lies in the very fact that their food and clothing are different from those of the Chinese, and they are therefore not dependent upon the Han for anything.

Now that last bit about them being “not dependent” on the Han Empire is baloney – the Xiongnü’s bread and butter – like its successor states – was to “trade and raid” its neighbors, chiefly the Chinese borderlands. But that population estimate, looking down its nose at the “barbarians” as it is, can nevertheless be used to be an idea of the steppe population under the Xiongnü Chanyus’ rule. We have much more complete information on the Han Chinese population, since they were diligent in maintaining their census records right up until the entire dynasty collapsed in the early 3rd century. User /u/Joltie and his peers put together a fantastic visual breakdown of the Han Empire ca. 140 CE here which displays the census data province-by-province. We can probably pretty safely discount the idea that our eunuch friend would have been including the border regions or vassal states in his eyeball estimations/comparisons, because frankly a Chinese politician working for the central government wouldn’t have given the boonies a second thought. It’s far more likely that when he speaks about a Han province, he’s talking about the Yellow River Valley region – the heartlands of China. And according to our data, that gives us a range of anywhere between half a million to about a million-and-a-half (with a few regions ballooning well beyond that).

That number does seem to jive pretty well with other ancient sources like Sima Qian in his Shiji, who routinely tell of the Xiongnu mounted terrors sweep into Han China with armies of hundreds of thousands of riders… now many modern analysts respond to any numbers bandied about by Sima and say, in effect, “yeah, just go ahead and knock a zero off whatever number Sima throws your direction.” The ancient Chinese (and ancient chroniclers in general) and notoriously bad at giving accurate estimations… still, it’s better than nothing. Furthermore, in this instance Sima Qian might have actually been closer to something resembling fact than he usually is with numbers. Aruz, Farkas, and Valtz Fino (2007) estimate the Xiongnü population at its height of perhaps as many as 1 million. Whoever is right, case it gives us an estimate from which to draw possible population distributions… At its height under Modu Chanyu, the Xiongnu Empire held some 1.5 million mi^2 of territory. If we put the number of Xiongnu at the lower end of the central Chinese provinces at ~500,000 then that works out to about 1 person per 3 mi^2. If we instead take Sima Qian’s number and go ahead and triple it to account for the fact that most of the riders would have wives and children not riding with them on raids (which puts it on par with Aruz’s estimate), that average distribution shrinks to 1 person every 1.67 mi^2. In either case, though, that’s a lot of room per person.

Now I know that I’ve just been throwing numbers around from a thousand years before the guy you’re asking about… but it’s likely that, all in all, even after a millennium the ability of the steppelands to support a given number of horse nomads wouldn’t have drastically differed between the 2nd century and the 12th. Unlike the Chinese, for instance, who were able to double their farms’ productive capacity by selective breeding of double-yield rice, and thus double their population in the 8th and 9th centuries, the life of a steppe nomad was pretty darned similar whether you lived in the year 140 or 1140. There was only so many people the climate could support. So even though the Xiongnu were gone for almost a millennia before the Mongols, their lifestyles were remarkably similar… and thus, we might expect the capacity of that same land would have likewise remained fairly stable.

OK, so let’s get past the preamble and look at some estimates of the Mongol Empire as it formed. We just don’t much of anything from between the collapse of the Uyghur Khanate in the mid-9th century and the rise of the Mongol Empire in the early 13th… the Uyghurs were shattered by the Kirghiz, and the whole region just went dark to historical record, sadly.

Nevertheless, by 1206, when Temujin was about 20 or so, and had newly unified a confederation of tribes including the Naimans, Merkits, Tatars, Keraits, and the Uighurs, under a territory roughly approximate to the borders of the modern Mongolian state (~600,000 mi^2). Estimates between 1204 and 1209 place the size of the army he amassed (and continued to amass) at ~20,000 at the time of unification, and swelling to some 80,000 by the time of the invasion of Western Xia in 1209. This of course does not take into account the noncombatants. If we once again take into account wives and families, estimates put the total population of the region just prior to the Mongol conquests as between half a million and a million. Ferdinand Lot estimated 700,000 in 1948, and that number remains thought of as pretty reasonable. So if we use it, that gives us 1 person per 0.85 mi^2.

As just a minor post script, the population of Mongolia today is just over 3 million people, and its modern borders are ~604,000 mi^2. Thus today Mongolia is MUCH more densely populated than in the past, and averages – get ready – 5 people per mi^2. Gasp!

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So, long story short: From time immemorial until essentially the advent of the modern nation state of Mongolia, the nomad steppe life seemed to support a small but stable population density in central Asia of on average 1 person per 1-3 mi^2. Nowadays they’re all packed together like sardines at an astounding 5 people per mi^2.

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References:

Aruz, Joan , Ann Farkas, and Elisabetta Valtz Fino (2006). The Golden Deer of Eurasia: Perspectives on the Steppe Nomads of the Ancient World.

Beckwith, Christopher (2009). Empires of the Silk Road: A History of Central Eurasia from the Bronze Age to the Present.

Hildinger, Erik(2009). Warriors Of The Steppe: Military History Of Central Asia, 500 Bc To 1700 Ad.

Kim, Hyun Jin (2015). The Huns.

Lot, Ferdinand (1948). L'art militaire et les armées au Moyen Âge.

May, Timothy (2008). “The Mongol Empire in World History”.

Twitchett, Denis C., and Michael Loewe (ed.) (1986). The Cambridge History of China, vol. 1: The Ch’in and Han Empires, 221 BC–AD 220.

Twitchett, Denis C., Herbert Franke, and John King Fairbank (ed.) (1994). The Cambridge History of China, vol. 6: Alien Regimes and Border States.

Watson, Burton (1958). Ssu-ma Ch’ien Grand Historian of China (tr. Of Sima Qian’s Shiji).

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P.P.S. Consider checking out This History of China Podcast! We're in the late Tang Dynasty of the mid-9th Century, and in the steppes the Uyghur Khanate has just been smashed to bits by the Kirghiz, leading to a crisis for the Tang when 100k refugees show up at their borders demanding to be let in.