I will warn you, however. I thought, "Hey, my computer doesn't have any issues with the normal max number of soldiers. I'll just crank it up to 800!" IT. WAS. HUBRIS.
Yeah.. I turned it up to ~400 when I first used it back in what, 2008? Back when the max was 150. I learned that isn't a good idea, just because the character models and graphics aren't amazing doesn't mean your CPU will be able to deal with all the people thinking.. Now even with my current pc I wouldn't try to push past ~250, since even the newest versions seem to be !!slow!! with 200+ people. Hiccups in the middle of battle isn't cool too [i.e. when 100+ reinforcements come there will sometimes be a lag spike that can get you headshotted], so I don't put it past 200's.
I think I had 275 set at one point, but it was hellish at times. I don't remember if the original M&B is more or less optimized for huge battles, but I still wouldn't recommend going past 200. 200 makes the warfare seem realistic enough, and your stacks are rarely larger than 100 in open field battles anyway, even late-game. So the epicness is there, and you still get waves if it's a massive battle in terms of numbers. Either way 250 is serious nosebleed levels and higher than that you're going to get hiccups because of the AI brains drowning your PC.
There is also the issue of M&B1 earlier versions having the 'ultimate blob of doom' sometimes where soldiers would just turn into a moshpit of anger and weaponry.. Probably had to do with me putting the limit too high, I think.. I got caught in one of those and my computer almost cried before I finally got plinked down to no health. I only recall this happening a long, long time ago. Not sure if it's as common nowadays as I don't play M&B anymore.
The AI thinking is rather processor intensive, seemingly. Something something something unoptimized code, they did clean it up though later on.