21 gear combinations (or 18 gear combinations). Some of these combinations will be identical, so you won't really have 21 different combinations. What you will have is 3 chainrings up front, and 6 or 7 sprockets on the rear wheel, and 3 times 7 gives you a 21 speed. You'll have less than 21 unique combinations, but you will have a useful range of gears for different conditions-- for example, low gears for going uphill, and higher gears for flat ground.