I voted 3 but love when you are down to 2. Then you must be ultra careful because losing one means ending the multi ball... no more room for mistakes.
What I would like to see more often is a PF multiplier (or jackpot multiplier) proportional to the numbers of balls still in play. That would give an incentive to keep as many balls as you can for the big jackpot. Otherwise you just feel overwhelmed until you are back to 2-3 balls.