I looked up 1 cup of regular soy milk and it is

1 cup (237mL)
Calories: 80
Protein: 6.7g
Carbohydrate: 4.4g
Total Fat: 4.7g
Fiber: 3.2g

So net carbs of 1.2 per cup. Looks pretty good for your latte if you find soy milk with these kind of numbers.

Just count your carbs... for the coffee, soy, sweetners....