Number of Units vs. Poly Counts

I have this idea for a game set during the Napoleonic period, kind of along the lines of the Total War series, at least for the battle sequences. Battles for this period could have around 100,000 troops on the field at a large engagement (counting both sides), but I figure even Torque might have a few difficulties rendering that many figures. What are the realistic limits on units in a map. The main thing is the rendering as they do not need individual AI or even movement, they will manoeuvre in large units.
I guess that reducing the poly count will improve performance, but is there a lower limit after which things dont get any faster (for example is a 50 poly unit much faster to render than a 100 poly).