How many dimples on a golf ball?

Exactly 420

See http://en.wikipedia.org/wiki/Golf_ball#Design

“Most golf balls on sale today have about 300 – 450 dimples. There were a few balls having over 500 dimples before. The record holder was a ball with 1,070 dimples—414 larger ones (in four different sizes) and 656 pinhead-sized ones. All brands of balls, except one, have even-numbered dimples. The only odd-numbered ball on the market is a ball with 333 dimples, called the Srixon AD333.”