Hi... As far as I know the Futaba FC-18 radio I own can be expanded with 2x8 additional channels. Both on/off or proportional. Each module will use 2 of the standard proportional channels of the radio to drive 8 extra functions. That means that you will have the normal four channels controlled by the sticks plus 16 additional channals. So.... 20 channals/functions should be enough.
The only drawback that I know of is that the 16 extra channels will be a little slower than the normal functions.