The choice of gears is probably the most important decision that a rider makes. Having to Push an expensive bike up a hill is embarrassing and usually very inefficient. Also it is a complete waste of time carrying a gear around that is so high that it never gets used.
Riders can opt for up to nine cogs on the rear wheel.
Combine this with a triple chainring and you have, in theory got a twenty seven gear groupset. In practice you will probably find that only about fifteen of these will be used. The remainder will probably be duplicates, or near duplicates i.e. different ways of making the same gear. The more cogs you put on the back, the more the wheel must be dished to compensate, and the narrower the hub flange spacing. The result is a rear wheel that will self distruct sooner due to increased stresses.