G'day Mate, it all depends on what you intend to get into in the future, for instance if you are going to fly larger aircraft, that use 2 servos for ailerons, & 2 servos for elevators, 1 for rudder & 1 for throttle, then the minimum would be 7 channels, that way you don't need Y leads, & you have great control over all your flying surfaces, remember that the 7th channel is for retractable wheels. So it all depends on how much money you want to spend & what you want to do in the future.
Good Flyin Mate.
Allan.