Not much...as they use liquid Splenda. Liquid Splenda, which in the US is only sold to businesses (such as Diet Soda makers) has virtually no carbs.

For comparison, 3 packets of Splenda has 2.85 +/- 0.15g of carbs...entirely from high-glycemic sugars...while a diet soda (according to fitday as I don't have an exact carb count) has .355g of carbs...which I believe mostly comes from the sweetener itself. Before, you assume this number as correct...I should point something out:

As I understand it, carbohydrates are calculated using a difference formula. This formula can overestimate the carb content of food. Basically, they calaculate calories, fat, protein, fiber, water, ash, sugar alcohols (when used), and alcohol content and then the rest is assumed to be carbohydrates.

Since, an artificial sweetener has no fat, protein, ash, water, alcohol, or sugar alcohols...it is assumed to be a carb...with the exception of Aspartame which includes two non-essential amino acids and thus has a fractional protein content in addition to a fractional carb content. Based on its sweetness, 75mg (0.075) of Splenda should be present in a diet soda. The other 0.280g must come from the preservatives, etc...

I would guess that in the end...there are probably 0g of real carbs in a Diet Soda. But, I'll stick with the .355 figure for tracking my personal carb intake. All in all, 3 diet sodas (other than aspartame diet sodas, which can spike blood sugar in some people) have about the same number of carbs as a packet of Splenda and only a fraction of the glycemic index if not a 0 glycemic index.