On American golf balls, there are usually 336 dimples, while there are 330 on a British ball. However these numbers can vary widely.

Most golf balls have between 300 and 500 dimples, which have an average depth of about 0.010 inch (0.0254 cm). The lift and drag forces on a golf ball are very sensitive to dimple depth: a depth change of 0.001 inch (0.00254 cm) can produce a radical change to the ball's trajectory and the overall distance it can fly.

Dimples on a golf ball create a thin turbulent boundary layer of air that clings to the ball's surface. This allows the smoothly flowing air to follow the ball's surface a little farther around the back side of the ball, thereby decreasing the size of the wake.

More Info: www.golflink.com