I've been listening to Hardcore History and there's one thing I can't get my head around. When I think of Steppe tribes I imagine a couple of dozen tents and 100 or do people living together, living off the land, moving around slowly to find food.
Not exactly the kind of scale from which you could even begin to build a feared army.
Were these tribes 100s or thousands of people? And if there were thousands then how could they all be fed without farming? The Mongols were hunters, not farmers. How could hunters kill enough food everyday to feed the town?
There are too many inaccurate presumptions here for this to be a valid question.
First of all, there were no Mongol cities at that time, the first city, Karakorum, was founded a decade after Genghis Khan's death.
And Mongols weren't hunter-gatherers (maybe some in the more inhospitable north), they were herders. They would hunt for sport/practice and to supplement their stores especially for the winter when animals can't produce much milk but they didn't rely on hunting for food.
In general the world's population evolved from hunter gatherers in two ways, though settled agriculture based around growing crops and through nomadic agriculture based on herding. Today nomadic herding is by far the minority but there was a point when it could be found across the world. In most cases farming was the better option but in highly infertile lands like the Great Eurasian Steppe, herding was far more efficient and allowed fairly high populations to be supported. For example, when Chinggis Khan invaded the Xia after uniting the tribes his army numbered 90,000 men. From that number we can see that they did not have an absurdly low population for the time, just a low one. I can not remember the number off the top of my head but I believe estimates on the Mongol Empire pr-expansion population are around 200,000 to 250,000 people.