It depends on the bike if the engine of bike is heavy it has more gears. Usually 5 gears are max gears.
click the B button to shift between 2 gears
There are seldom only 2 gears on a bike. Many have 12, 18 or 21 .
an average bicycle has up to 24 gears but a bicycle has up to 30 gears
MotoGP bikes have 6 gears.
depends on the model. from 2-7 gears
Depending on the bike yes most adult sized bikes have gears
if you don't keep your chain and gears on your bike greased it will rust and come off.
A good bike today is most likely to have 11 gears(MTB) or 22 gears(road).
a track bike, or a bike with internal hub gears.
Unfortunately, you can't change gears on bikes in HeartGold.