Yep, you need to do all ten bets. Always do all ten from the same person and it works best if you stick with that person for the long haul as different betters have different rates of winning and if you try and pick and choose you can find yourself losing a lot.
When the opening odds come out some arenas will be positive - so they have one 2:1 and three higher numbers (7:1, 13:1, 13:1 for example), meaning there is a decent chance those 2:1's will win. The higher the other numbers, the better the positive.
Other arenas will be 'boosters' - a spread of lower numbers like 2:1, 3:1, 4:1, 7:1 - giving them a chance of winning bigger when you add them to the positive.
Bets will use the positives arenas as the 'base'. Decent booster arenas are then picked, so seven bets for instance may go to boosting two arenas (say a 2:1, 3:1, 4:1, 8:1 and 2:1, 3:1, 3:1, 13:1 - the 13:1 doesn't get a bet as the chances are so low). Remaining bets will be used on safeties (you want to cover any of the non 13:1 pirates usually to cover your bases) and if there's a bet left it will go on a base bet (the 2:1's from the positive arenas alone).
If you don't use all ten bets, you significantly reduce your chances of winning anything.