I can't help feeling that the original post is incomplete. While I have shown how to control 12 Servos using the Arduino Servo library, I have not provided any indication of the performance impact. If it takes 90% of the available processing time to control the 12 Servos, using a single Arduino is not a very practical solution.
Its not that big problem either, anything that requires 12 Servos could probably benefit from the additional processing power and sensor inputs that one or more additional Arduinos would bring. When an additional Arduino can be built for less than 10 dollars there is no good reason not to take this approach.
Anyway, in the interest of finishing the original post and learning a little more about the Atmega architecture, I plan to follow up a few ideas I have for using Timer0 and a set of loops to measure the impact of 0,1, 6 and 12 servos. At the moment I am several thousand miles from home but will get onto this in the next few days.