The total "number of gears" isn't the issue, the Range of gearing is.
The answer is both "it depends" and "it changes".

A cyclist requires a Low range "bottom" gear that allows climbing the worst grade they will encounter comfortably and a High range "top" for bombing down the backside w/o the "hamster-in-cage effect". Cram as many cogs on in between as budget and aesthetics allow for efficiency.

Required ranges will change w/ terrain, weather conditions, load, distance and fitness.
Modern cassettes are easy to change and relatively inexpensive. Swap out a straight block "flatland set" or a medium range "hilly" cassette to a wide range "mountain" as conditions require.

Set-up your machine w/ the chainrings (one, two or three), derail type and shifters that works with your Widest Range, swap cassettes as required and proceed.

-Bandera