With the exception of radios designed for park fliers, modern radios have a range which exceeds your range of view of the plane (unless you use binoculars). Park flier radios typically have a range of 600 feet. I once read the range of a standard radio is approx 1 mile, but have never been able to confirm it. The farthest a plane ever got away from me about ¼ mile, and was nothing more than a dot in the sky.

You can use any system with AT LEAST as many channels as you need. I.E. if you need 4 channels, you can use anything from an inexpensive 4 channel to the highest end unit (24 channels I think).