My 2M pattern ship uses 6 channels, My 35%'s use 9 channels, even my small Stream 50 uses 6 channels. In fact, the only plane I have that uses just 4 channels is an indoor electric.

Helis really need 5 channels. A good sailplane with flaps, spoilers, crow mix, etc will need at least 7 channels. Talk to pilots who fly turbines and they'll tell you 12 channels aren't enough.

You can fly lots of planes with just a 4 channel radio. You can even fly dual aileron/elevator planes with a y-harneses, etc. But more complex planes really benefit from (or may even dictate) more channels. Much easier to set up, more flexible, not to mention the higher channel radios have more mixes.

For the average powered sport pilot, 6 channels is probably all they will ever need. Throttle, L-aileron, R-aileron, elevator, rudder, that's five, then six could be used for flap or gear retract or second elevator servo or smoke or etc.

Cheers