6 channels has lasted me for quite a while flying airplanes and helis.
I am wishing I had a 7 channel now for a scale heli with retracts. My only option with the 6 channel radios I have if I want retracts on a heli is to go with less of a gyro (gyro sensitivity is controlled on channel 5 which is also the gear channel)
Of course the argument could be said, should I really have any need of remotely switching the gyro in a scale model anyway?
So my answer is, get at least a 6 channel radio and 7 channels are better (make sure your radio has heli programming with several swashplate options and 5 point pitch and throttle curve settings)