I played on Veteran difficulty, and I had 143 little dragons join me! I was so surprised by that number! I've never had so many dragons under my command before. I also completed it using this guide. In the last week, when that Thrallsai appeared, I had to reload my saves many times to make it in time to visit the last two upgrade locations with Ilay, and in the end, that jerk didn't even attack my Castle! I had to chase after him.
And here's the army I attacked him with: D
Me - 448 Assassins - Computer - 712 Assassins
Me - 339 Furies - Computer - 340 Furies
Me - 368 Minotaurs - Computer - 230 Minotaurs
Me - 138 Lizards - Computer - 100 Lizards
Me - 170 Black Dragons :)) - Computer - 0 :D
I defeated that ... jerk in 2 turns. He did manage to cast Armageddon, and that was his last word.
The number of neutral creatures may also depend on the time, meaning how much game time has passed since the beginning of the mission.
Now I'm using a mod that allows me to play all Campaigns with Patch 3.0, so there might be completely different numbers and situations.