There are 11 horse races in The Witcher 3: Wild Hunt. Winning all of them unlocks the Fast and Furious trophy / achievement.
Doing horse races is well worth your time! The saddlebags you earn will let you carry up to 160 items in your inventory, the racing saddles will give your horse more stamina and with the blinders your horse won't get frightened by monsters so quickly. You will also earn a great deal of money for very little work (the 7th race alone gives you 700 coins and takes only 3 minutes). It's a good way to get some cash early in the game.