None. The difference between 130 grams and 117 grams is easy to detect manually without weighing.
Otherwise, the answer is one. Is there a way to do hidden spoilers on here?
Number the piles 1-13
taking care to keeping them from mixing, take one coin from pile one, two coins from pile two, three from pile three, and so on until you take the entire 13 from pile 13 an place them all on the scale.
The difference in the weight from 910g will tell you which pile has the phonies (e.g., if the pile weighs 908 grams, there are two coins off, so the impure are in pile #2).