Theory: 2 characters per server minimum
I realized that it is imperative to have 2 characters in the same faction for each server. Otherwise, who will be the holding pattern for stuff? I had to create a new character on Kirin Tor because Crixikuhn is in Auberdine, far away from the Auction house, and it's not very convenient to get to the bank all the time. I've known this for a while, but it wasn't really in my plan to create another character.