We will start with an 8-channel model, but a future model could offer 4 channels only. Having an external screen would make the design a lot more complex. The beauty of using an iPod Touch is that it gives you not only a fantastic screen, but also a multi-touch interface, a powerful CPU, 512MB of RAM, 16GB of flash, a camera, a WiFi interface, a great operating system with a huge developer community, and many other goodies, for just $199. It's pretty hard to beat...
The unit is 42HP. We do not know yet if we will manage to cram all components onto a single backplane, but we're pretty hopeful that we will. If so, it should be pretty shallow, allowing you to stack multiple breakout boards on top of each others. When doing so, stacked boards will share their pin sets. And to be clear, all breakout boards are connected onto the module's backplane, therefore do not increase the amount of horizontal space you need to dedicate within your rack. 42HP is all you need for the submodular synthesizer and 8 1S breakout boards.