There's no one right answer to this. It varies depending on your cat's age, activity level, what type of food (you said can so I'm assuming wet, but do you also leave out dry and do you give treats?) and it's contents (high carb or heavier on protein?) you're feeding, or if you're trying to maintain weight or need to lose or gain weight. Can you give some more information about your cat and what you're feeding?
Thanks, that's helpful. He should be getting towards the higher end of 25-30 calories per pound of his body weight, so if he's at 15 lbs, he's going to need approximately 450 calories a day or 7.2 oz on the high end. If he's closer to 10 lbs you can probably get away with a 5.5 oz can a day but depending on what's in it, your cat may still feel hungry.He is on all dry at the moment.Hes pretty active and is around 2 years old. He needs a little more weight but not much.I havent bought any wet food yet,I have been reading that its better to get a cheap wet food and a more quality dry?Thats what I am trying to do,or if I can,just feed him wet.