No, all horses don't have to win in an Accumulator. However, the more horses you pick that do win, the higher your potential payout will be. That's because each horse that wins adds another "leg" to your accumulator bet, and each leg has its own odds. So, if you pick four horses in an accumulator and three of them win, you'll still receive a payout according to the odds of those three winning legs. Of course, if all four horses win, your payout will be even higher!