The normal servo library can control up to 12 servos. There are libraries available for controlling up to 20 servos. I don't understand why you'd want to use an external controller.
I gave some reasons why I don't like to use servo control boards in this thread. Here's part of my reasoning.
I generally don't want servos moving at constant speed. In the joystick/pan & tilt example, the program accelerates the servo up to speed and decelerates the servo to a stop. The target stopping point is constantly changing as the joystick is moved. So even if the servo controller could handle the acceleration and deceleration (which I don't think it can) the parameters of target position and target speed are constantly changing.
Besides the joystick/servo videos I have a couple other servo videos which demonstrate motion I don't think servo controllers could duplicate.
This is a simple demo showing a normal servo sweep with the position incrementing in a linear manner (servo on the left) vs a sinusoidal motion (servo on the right). I believe only the linear motion could be produced using an external servo controller.
This video is similar to the one I just mentioned, but on a larger scale. By controlling the position of each servo at 50Hz I think more complex motions are possible than simply commanding servos to move at a set speed.
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I'm sure there are plenty of applications where a constant velocity servo performs the desired task just fine. I just find I usually want a more complex motion than an external servo controller can provide.
It would probably be a good idea to let us know what you're trying to do. How will these servos but used?