RE: transmitter channel question
You don't. 4 channels will fly a typical fixed pitch helicopter. 5 or 6 are most often used for a typical collective pitch heli.
Where the 9 channel transmitter comes in is in the advanced mixing and programming capabilities that a higher end transmitter has.
Being able to support various swash plate configurations, CCPM mixing, throttle and pitch curves, revo mixing, all of these let you do more with the RC heli hobby.
Other features like end point adjustment, model memory, dual rates, exponential and such are added bonuses.
Additional channels make adding retracts or onboard cameras doable.
The common recomendation is to get all the transmitter you can afford.