Many times "channels" are used up with mixes, not separate functions of the airplane. For example, I have an ultrastick that has flaps. According to the above calculations, I would need a five channel radio. While five channels would make the plane fly, it would not allow me to utilize the "crow" function to it's fullest. For that, I had to have at least seven channelseven though I only have five separate functions on the airplane.
Personally, I would not buy anything less than a seven channel if I could afford it. I owned one four channel, and two six channel radios before settling on a seven channel. I've never been unhappy with the seven channels, but I was always wanting a seventh channel when I had the six channel radios.
JMO.