If you're having trouble grokking channels in AE2, there's a really good video explaining them that I link to all the time right
here, on the AE2 wiki.
Channels are indeed a bit confusing at first. One way to think about it is in each line of regular cables, there are really only eight individual wires that can be used to transport data. Each machine has a single port where one of these wires can be plugged in, as well as seven more spaces for wires to just pass straight through the machine and out the other side. Since each normal-size cable bundle only has eight wires, you can only connect eight machines to the controller through it, because you can't plug multiple machines into any individual wire because that would cause interference and break things.
With this visualization, it might help to think of each machine not as a solid block with ports on each face, but rather as more of an open box with a single port to connect it to a controller and enough space inside to run several more wires through. There's no way to even connect multiple machines to the same wire in this way, since you can't have a wire with more than two ends, and one of those ends has to plug into an ME Controller.