How many saltwater fish can a 150 gallon fish tank hold?

The rule is 1 inch of fish needs at least 1 gallon of water. A tank usually has an inch or two of gravel at the bottom and is usually filled to within an inch of the top so if the tank is 12 inches deep approximately one quarter of it is not holding water. Now in your case you would be lucky if your 15 gallon tank is holding 12 gallons. So doing the simple addition it looks as if you could have up to 12 inches of fish in there. So any combination will make it up ie 1 x 12 inch fish, 2 x 6 inch fish, all the way down/up to 12 x 1 inch fish.