Given that's the steppe region without much in the way for farming or long-term settlement the population would most likely be lower than normal. Though you're a bit on the big side ~1m would have been an average. Though also given the area I doubt there'd be much going on to support a static population a quarter or half that. Most of anything else would be nomadic bands just moving through with their goats or horses or what not.

So a few hundred thousand maybe spread between a few permanent settlements and the rest moving around would be my best guess.