My own personal experience - not based on a poll or knowing Blizzards higher standards - is that around 50 to 60 units the AI or the computer starts having issues and units start standing around or being ignored by commands, missed by "picked unit" etc.
Around 45-50 things start to slow up on a "regular" computer. Aka "lag" even in "leak fixed" maps. The processing of that much information is going to lead to "issues".
There is probably a good reason why food limits are set at 100. Considering that peon types have one food, Footmen use 2, Knights use 4 - and heroes can use 5 we can quickly surmise that there is no way that blizzard was using 100 units for say the melee AI.
Try an experiment - run one of the melee maps by blizzard with all 12 teams, push the food limits up to 150 or more and you can pretty much "feel" the pressure as more units are in play on your machine.
If you look at the campaign maps of Blizzard you find that they tended to keep the number of active ordered units low. The heroes went up against small groups - its not until you get to the melee type maps that a higher number of units are on the map.
There are "arounds" for this sort of thing.
1. Make units with a higher acquisition ranges to where they can "see" and target enemy units further. Not too high, say around about 1200 to 1500 (around about 3 large grids)
2. you can give units the wander ability to where they wander aimlessly about.
These two will present you with more units to attack and show up here and there (I assume zombification means that these undead wander about already).
Basically have two forms of enemy unit - ones that wander and attack at random, then a small core of units which are part of your unit groups that you send with orders that are "intelligent" and are being used effectively with triggers.
It would be easiest to just add wander ability to your extra units via object editor and not trying to pick all of them and add the ability as they are created or as they are located on the map, making customized units for that, then keep your default units for being ordered about the map.
Since you are using 10 players that only leaves you with 2 computer controlled teams. I would divide up the "work" of the creeps between those two teams, and use Neutral Hostile and Neutral Extra for the randomly wandering undead spreading out the load as much as possible.
Of course you are making a game for use by people who may not have 1GB to play with, or with the latest software and hardware - if at all possible I would strongly urge you to test the game on a machine with less power and see what happens.
