All depends on how you want to build your gameplay. It can be 2 or 3, but for sure not 1 and not more than 3.
Regarding the map divides, the current standard is to use the best possible place on each maps. Regardless where it is located on maps, and how you want to divide the empires.
If the main goal is, to completely separate the empires, you need to rework all the current maps a bit, also you have to completely rework every regens. From a player perspective, I wouldn't want to be killed all the time on low levels, so it would make sense to separate maps in low level, and let the players fight for good places on higher levels. But as I said, it's mainly a gameplay question. But if you keep the concept original, for sure it's better to stay with 3 empires. I haven't seen yet any server to work on empires as the part of the gameplay, so I can't tell you an example. But the empire specific events would be a problem I guess, as lot's of people in the game is in guild, or married, so they can't change empire so easy. And those players for sure would questioned the higher rates or empire specific events. Players only care about the best available options, in every segments of the game. It's not the same game as it used to be, players are not exploring anything, they just rush as fast as possible. So if a certain empire gives even 1% advantage in anything, you can be sure that 90%-of players will go there. But i'm still kinda optimistic, that with a good gameplay plan you can kinda balance out the empires. But let's be honest, private servers don't give a shit about it, it takes time and tests. And players don't care either. So why would they care?