How much canned food should i feed my cat?

I have to switch my diabetic cat from dry to wet food to lower carb intake. He's 10 years old, 18 pounds, and a little underweight. He's a very big cat, possibly part Maine coon. I can't find a feeding chart for canned food. He eats twice per day, around his insulin shot. How many cans of science diet perscription m/d 5.5 oz per day should he eat?