How much should my cat eat?

My cat is about 11 lbs, he's a big guy, and a maine coon. He's not fat though. I just adopted him, and he seems to have a huge appitite. I feed him a serving of wet food that is supposed to be a 3 serving portion, but he eat this much in the morning, and then in the evening starts meowing that he wants another one, but I don't want to make him fat. How much should my cat eat?