I've been flying rc planes every year since 1979. I've had 6 channel radios for the past 15 to 20 years figuring I'd need the channels. About five years ago I started using a fifth channel for dual aileron servos. Looking around our club, we once had a guy who used retracts. Several have used flaps. Other than that everyone is flying on four channels, or maybe five for dual aileron servos. Actually I sometimes fly dual ailerons on four channels using a Y splitter, and some others do too. Many have 6 to 10 channel radios. What do they do with all those channels? Beats me. But the computer radios have more features in the 6 channel sets than in the 4 channel sets, so I stick with 6 channel radios, but if I didn't have those features I'd still be flying the same planes just as well.

You should definitely have at least four channels. Beyond that, it really doesn't matter much. Radios are cheap, and they have been getting cheaper pretty steadily for about 50 years. Get one you can afford and see if you need more channels later. Don't worry about what you "might" want later.