Have to control 13 motors independently and not in master slave modes. Must run through one software solution were they can all be controlled. Motors are Nema 23 steppers. So need to find a control board and drivers. Found one but it was $17,000 USD !!! There must a control system out there that is like the cheap Arduino system but can handle more motors and comes with software and hopefully has an opensource community behind it. So strange that once you go beyond say 4 motors it's like you are talking about building a warp capable spacecraft that has never been done before and why are you doing this etc.
I believe the Arduino Mega has enough IO pins to handle that. Add 13 stepper controllers w/external power. The "comes with software" will be the tough part. Depending on what you want to do (robotic motion, cnc, pnp, Iron Man suit*, etc), you might be able to find some software that you can extend/modify. I would definitely expect to do some coding. Arduino forums might be a good place to search. (* 10 stepper motors & Arudino Mega )
