Channel 1 Throttle
Channel 2 Ailerons
Channel 3 Elevator
Channel 4 Rudder
Channel 5 Retracts
Channel 6 Flaps (or flaperons, two servo ailerons)
Channel 7 Wheel Brakes
Channel 8 Wheel steering seperate from Rudder operation
Channel 9 Speed Brake, smoke, or other BS
Channel 10 I have two rudder servos that are in two channels that can be mixed together. On the JR 10X system, you don't actually do this mix on channel 9 or 10 becuase of a refresh on the channel time difference, but it's certainly another channel option. This list could actually go further for instance
Channel 11 Canards
Channel 12 Independent Canards
Channel 13 Drop Bombs
CHannel 14 Independent control of gear doors?
Want a camera? Channel 15

Just depends on what you want to do. I could fly "jet" on four channels. Take a Top Gun F-15 Ducted fan with no remote control of the needle valve on the fan unit and no retracts. That's a four channel airplane all day long. Sky's the limit (pun intended) but actually limit is 10 channels on 10x and I heard that airtronics has a 13 channel, but take that as a rumor unless someone else chimes in as I didn't actually count the channels when I saw their top line transmitter. I just overheard someone say it was that many. As I posted this, I see someone else tackled the question and put things I didn't think of like Gyro Gain sensitivity, that's a good one. There really are a lot of options when thinking about channel usage and different radios. For starters, just go with a good JR 8103. That will go all the way through a good running Kangaroo turbine.