It really, honestly, depends on what you'll need. My dad has a Spektrum DX7 because he wants to have the ability to use flaps on his sailplanes, and the ability to use telemetry.
I'm going to 3D print a P-38 Lightning from the 3DLabPrint guys, and with that, I'll need a minimum of 6 channels (Throttle, ailerons, rudder, elevator, flaps, and retractable gears). If I wanted to throw in dual ailerons rather than using a Y-connector, I'd need a 7 channel transmitter.
Will you use a ton of switches? Most likely, no. I find I use, at most, 3 switches. I have a throttle kill switch (something my field requires I have in use for everything I fly, with the exception of sailplanes that have no motor), a switch for various levels of expo, and a switch to turn on mixing for my differential thrust on my Sea Duck. I suppose if I had another twin motor plane that I wanted to set up differential thrust AND have retractable landing gear, I might use more than 3 switches, but honestly, it's kinda rare.
I'd recommend planning for at LEAST a 6 channel radio; while you technically can get by on 4 for most planes, you'll find that you need at least one dedicated channel for that throttle cut, and having another channel for changing flight modes is a bonus for drones. I'd say that it really depends on what you plan to fly; if you think you're going to get into helis and intricate gliders and wanting to open a bomb bay door while retracting landing gear, it might be to the benefit of purchasing a higher channel radio.