I am looking for a substitution for milk in risotto
So the milk does 3 things: 1 adds dairy flavor, 2 adds liquid, and 3 adds fat. Maybe if you mixed 2/3 fat free sour cream with water until it was a consistent texture, you might have a kinda-approximation. It'll be thicker, perhaps granier, and tangier. Personally I'd just add 3/4 that amount of water and 1/4 that amount of butter.