I read that you have only 1 sneak attack damage per turn but sneak damage gets multiplied on a crit.
I also read its once per turn, which means you can add sneak damage only once when you attack on your turn, but you can add it on another characters turn. Some classes have an ability that allows another party member to make an attack or enemies provoke AoO. Of corse you still need to have advantage or an ally stands close to the enemy.
You MUST have a ranged or finesse weapon, it does not work with other weapons.
A ballista is a ranged weapon, right
Prof. Dr. Dr. Mad S. Tist 
