There are 11 horse races that you need to win to earn the Fast and Furious trophy / achievement. 3 in Velen, 4 in Novigrad, 4 in Skellige. It's recommended you do them in the order shown in this video as some races can only be played if you won certain other races already. The ones in Velen are all at the same location and the track is the exact same. The first 3 of Novigrad are also in the same place, but each one has a different track. Keep talking to the quest giver to ride against the other opponents. The ones on Skellige all have their own unique location.

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.