The right number of gears is determined by 2 things:
1. How easy do you need your low gear to be and how fast do you want your high gear to be? As I have aged, I have found myself looking for easier low gears and never using my fastest gears. YMMV.
2. How small do you want the steps between gears to be? I like the gears that I use on flat roads to be pretty closely spaced but I like bigger steps between my hill climb gears.

The trike that I ride today has 30 gears. I rarely shift out of my front chainring so I'm pretty sure that I could be happy with just the middle ring.