Answer:
There are many kinds of dunks. Let's assume Kobe Bryant a simple dunk - one made by standing directly under the basket and jumping vertically for the stuff. Since Kobe's hand touches the basket after he jumps, we have to compute for the distance between the ring and his vertically stretched arm.
10 feet (height of NBA basket) + 6 inches (extra height necessary to stuff the ball) - 6 feet (kobe's shoulder height) - 2 feet 6 inches (kobe's vertical arm length) =2 feet.
Kobe Bryant jumps approximately 24 inches to make a dunk. Add a few inches for the more spectacular dunks where he has to jump a little higher.