The Sneak Attack description specifies that you can use the feature once per turn, but it’s not limited to your turn. The feature also doesn’t limit the number of times you can use it in a round.
You sometimes get a chance to use Sneak Attack on someone else’s turn. The most common way for this to happen is when a foe provokes an opportunity attack from you. If the requirements for Sneak Attack are met, your opportunity attack can benefit from that feature. Similarly, a fighter could use Commander’s Strike to grant you an attack on the fighter’s turn, and if the attack qualifies, it can use Sneak Attack. Both of those options rely on the use of your reaction, so you could do only one of them in a round.
Because you get only one reaction per round, you’re unlikely to use Sneak Attack more than twice in a round: once with your action and once with your reaction