If Mia wants to use 6 cups of rice, she would need to multiply the amount of each ingredient by 3, because the original 2 cups times 3 is the 6 cups she wants to use. So the new amount for the recipe would be this:
6 cups of rice
12 cups of milk
1 cup of sugar
3 handful of raisins
3 cinnamon sticks
When asking how many this will serve, we also multiply by 3 the original amount of people which is 4 to get 12 servings of Arroz con leche