I recently started my final conquest run on a huge map. I had 150 cities at the start, with about 2400 ground units, 1500 air units, and 1200 water units between them for defense. I don't like having to worry about shuffling units around as enemy landing fleets show up (I like to kill them as they land, so they don't pillage the hell out of my countryside), so each city has pretty much the same defenses.
I have three major attack fleets, each consisting of 10 Battleships, 10 Destroyers, 10 carriers, and 25 transports. In the transports are 4 Gunships, 36 SEALs, 20 MAs, and (at the start) 40 Mechs.
There were four smaller fleets wandering around to conquer isolated islands (the main fleets go after the archipelagos), each with 8 warships, 12 MAs, and 4 Mechs.
Each main fleet has 200 Stealth Bombers to follow them around, while the mini-fleets have 100 SBs to provide support for one at a time. I also had 200 SBs on the biggest island shared by my cities and my most powerful enemy, to soften the dozens of units he'd be sending into my territory (there were a few dozen of my own waiting solely to hold them off, aside from my standard city defense). This is Warlords, of course. In vanilla Civ4, half the number of bombers would do the same job, since Warlords severely nerfs collateral damage.
Overall, probably around 7000 units, with the military rating at somewhere around 150M. The largest enemy had a 31M rating. I'd estimate that overall, there were around 10,000 units on the map across all remaining civs.
The game does not handle that many units especially well. It seems to actually try to render every single unit, even though only one should be visible per city (if you toggle bare map, suddenly everything's speedy again).