Every attack made against a target that is either flanked or denied their dexterity bonus to AC is a sneak attack and applies the damage. There is absolutely no per-round or per-day limit.
Example Situations:
A Rogue with TWF enters combat and wins the surprise round. He charges and makes one attack (because it's a charge) against a surprised opponent. This is a sneak attack because the opponent is flat-footed, and therefore denied Dex to AC.
The Rogue wins initiative and attacks the opponent again, this time twice because it's a full attack. The opponent is still flat-footed, and denied his Dexterity to AC against both attacks. Both attacks are, therefore sneak attacks.
The Rogue gets Invisibility cast on him and attacks his opponent. The first attack is a sneak attack, because his opponent can't see him and is, therefore, flat-footed. The second attack is not, however, because the first attack ends the Invisibility effect after it is made, and the enemy can now see the rogue.
A Fighter then moves up to flank with the Rogue as the Rogue's turn comes up again. He makes his two attacks and both are sneak attacks, because on each attack the Rogue is flanking with the Fighter.
