In terms of channels. Well you always need throttle so that's 1. on the real cheapos the 2nd channel is rudder. on a 3 channel they add either elevator or aileron. on 4 channel, they have throttle, rudder, aileron and elevator. on a 5/6 channel you have two aileron servos (3 total on cyclic), rudder and throttle, the 6th is used for a remote gyro. The 7th is typically used for a governor on nitro helis. Anthing above that is usually for other add ons like lighting, retracts... etc.