
Kolokotroni |

So long as each attack occurs within circumstances that would grant you sneak attack, then there is no limit.
For instance, if you are flanking an opponent, you can sneak attack for as many attacks as you make, including things like an attack of opportunity. If a target is flatfooted because it has not yet acted, same thing.
If its because you are in stealth, or invisible, then likely the first attack breaks that (its hard to attack and then hide via sniping and unless its improved invisibility, attacking breaks invisibility), in which case you only get a single sneak attack.