Sam Yikin on 21 February 2010
From Halopedia:
Halo: Reach's engine will be able to support up to 40 active AI characters and 20 vehicles on screen at once. In addition, faraway battles between AI characters will follow automated behavior models, which convert into scripted behaviors as the player gets closer, and finally full AI.[8][9]
So i highly doubt that there was a tenfold increase in supported AI since this was written (around the time of the GI article I think.)
Though 40 AI and 20 vehicles is still damn impressive.
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