Carbs in a skinny margarita can vary widely depending on the sweetener that is used, and how much. Often times, restaurant versions will substitute agave in place of simple syrup, which is natural but still loaded with sugar, so skinny margaritas can have upwards of 30-40 grams of sugar per serving.
In contrast, my low carb margarita recipe has less than 1 gram of carbs and 0.3 grams of sugar per serving!