Short answer, 7 on a huge map around 80% of the time. Long answer follows.
For discussion purposes here, assume large map for less than 5 computers, huge for 5 to 7.
I used to use the "rush to tool, wall up, tower up, and repair the heck out of your stuff and wait for the opponents to run out of gold" strat. I could usually (around 80%) take 4 hardest computers this way. I had beaten 7 but my rating was somewhere around 1%.
Ender clued me in that I was "cheating". The computer would never wall, and the AI couldn't deal with it, so walls aren't fair. I learned the "rush to tool, spread out all over" strat, and my success rate went up. Maybe diverting villagers from econ to defense was hitting me too hard. Without changing anything other than that, I suddenly found that I was able to beat 5 about 75%. Exploiting the Suicide Repairman AI gave me about 75% vs. 6, 20% vs. 7
Noticing the computer posts an 8-12 min bronze, and that they don't forward build, I started the "rush to bronze, then spread out" The computer usually attacks your outposts, destroying the granary or SP before moving on. Sometimes you can get the computer to follow a villager on a wild goose chase for about 4 min! If the computer heads for your base camp, put military buildings in his path. They provide too tempting of a target for him to pass up. You are just trying to buy time for the bronze ticker to get to about 60%. Unless he has ST, the TC won't come down before the bronze completes. Then boom from your new TCs to your heart's content, and you win. My record is currently around 80% vs. 7, and 95% or so vs. 6.
The reasons the strats work:
- The computer will not build more than 50 tiles from his TC, based on StoragePitMaxDistance and GranaryMaxDistance, so his collection rates bite in endgame.
- The computer only has instructions to build to 50 pop limit, and doesn't go beyond that point, so his econ is easily outdone.
- The computer will eventually run out of gold. Protecting gold sites is game-winning, because the only infinite rebuilds are gold-based units.
I suppose to be "fair" I should set the 50 pop cap too. But I am not good enough to do that yet with any non-discouraging success rate.
Keep your stick on the ice.