Open-source real-time strategy gaming with 0 A.D.

I was responsible for balancing an RTS game with seven factions, based not on empirical statistics but on player feedback and my own personal experience. I think I did fairly well, no small part due to luck, but success depends on your goalposts: Blizzard's standards (each faction has an equal chance of beating each other) are dramatically tighter than mine were (each faction feels as if it might have an almost-equal chance of beating each other). Four factions would be significantly more difficult to balance than three; six or seven are drastically more difficult. It had not crossed my mind than anyone might even attempt twelve, but I wasn't familiar with Age of Empires or Rise of Nations.

For an example of a game that successfully pulled off nine factions, look at Kane's Wrath. Here, there are three main factions, and the other six are just variants on the original three with a few changes. If 0 A.D. has taken this route (I still have not played it) then they have a better chance. Another possibility is that they simply do not care about balancing everything well, and just want to make a really cool RTS, which is fine as long as they're not trying to build a traditional competitive RTS community.