When complete fog is enabled, does the AI know how many armies are in the territory it is attacking? If not, can the AI ever deduce it from the history?

The AI always plays with normal fog, no matter the settings.

So sometimes it's helpful in complete fog games to be replaced by an AI and look at its orders to see how many armies are in the territories you are attacking? (The AI usually attacks with twice the number of defending armies)

In every game with more than normal fog, you could use this yeah. Nevertheless it isn't such a big deal, and I can only imagine it being a possible advantage in a few uncommon scenarios. The AI might trick you into assuming incorrectly also >it doesn't account for deploy and does relatively random ataccs (unless ataccing opponent/completing bonus).