DGUY:

As many channels as you can afford +2.

Some will insist that you do not need many channels initially, this is true... but a good TX tends to last a long time and during that time you'll have use of a quality piece of hardware.

The extra channels buy you other things in addition to the channels, such as more model memory ( more planes ), more mixes, more switches, etc.

Sooner or later as you move along in this hobby, you will find that you need as many channels as you can get...

e.g. one for each aileron, one for each elevator, one for throttle, one for choke or ignitor, one for smoke, etc. etc. etc.