Wasn't there another thread on unit numbers... "Magic number 24" or something?
Anyway, having more units under your control would insta-fail the multiplayer aspect of Empires. I couldn't imagine people playing real-time with no pause trying to get 5 units of cavalry to flank on one side and another 5 units to flank on the other whilst ensuring that the already engaged 20 units of infantry aren't flanked themselves. Of course, I doubt this player would even have thought that his 10 other units of reserves are being slaughtered just across the woods. I also doubt he'd have much time to admire the scenery, the funny hats, the detailed horsies and the cool ragdoll physics of his Lights whilst they are ambushed and promptly slaughtered en route to his main force.

That is of course if anyone still plays MP. Do they?