I've had over six thousand men on the battlefield at once, well I only had 1800, the Timurids had the rest.
This was played on huge units setting, with the reinforcement limiter off. My framerate was 20-40.
I don't see why it should be limited, given a choice of settings as we have.
However, I have never since played with huge units again, as the game makes them hard to field, and pathfinding really goes out the door..