Ive ridden bikes with 1,3,5,10,12,21, and 24 gears.
In my experience, in very hilly terrain, you dont really notice the extra cogs beyond 21 gears for everyday utility riding. The most important factor is the lowest gear you have. Most bikes come with plenty of duplicate gears, and plenty of gears too high to be of much use.
9 speed is just a marketing gimmick, which you pay for with cogs and chains that are too thin, and cog spacing so close that the gears need finer adjustment.
You also need wheels built with too much dishing to make space for all the cogs.