Every cat is different. My Niko is fairly active, 10 pounds (indoor cat but will play during the day) and I give him 2 cans of Fancy Feast daily (the small cans). It keeps him at a good weight. But, one of the things to consider is that unregulated cats can be unusually hungry, as their bodies aren't processing the food well. When Oliver was first diagnosed, we were at 2.5 - 3 cans a day, but when his numbers came down, we went back to 2 cans (which was his normal diet also.)
Remember the seafood warning - too much isn't good. IAnd if it is just people tuna, it isn't a balanced meal. Did you get some canned lo carb? How about trying some of it? If he has a weak stomach at all, stick with one type/brand until you're sure he is okay with it.
As some other kind person once told me when I was new and scared, there are no silly questions. We have all been where you are - it is a steep learning curve and you just can't get everything at once. Everyone who answers your posts is playing it forward for the kind answers they received.