600 soldiers total means 300 vs 300, and you can't see where the enemy soldiers are located. You could start an online game and join in on another PC on the other faction and calculate them all. Calculating the soldiers using the map can also be a bit difficult as the numbers get mixed up with base names and may become hard to read.
Also note that at the beginning of a game 80% of all soldiers are created.
After that they start killing each other and dying and spawning frequently so it might very well be that you won't hit absolute 300+300 at any point. If you did, the AI spawning would cease.
Anyway, don't play with the 600 setting, the game is just awful with that and might never cover that case in a way that makes sense, I really should put a mention about it in the menu

100-150 is well enough for the maps with few bases (map2, map3), map1 is good with twice that max. The AI doesn't have much organizing logic on team level, so with too many teams they just crowd the place and go back and forth and jump on and off things.