I would plant no more than two plants. In reality, a single plant can be fine for this size grow bag, but if you plant two, you can kill the weaker one once they start growing faster. This minimises your risk of having a bad tomato hog an entire bag for a season. You can expect a large crop of such a grow bag, if you grow the right tomato cultivar. This will depend on your area, but I would be rather disappointed if I did not get 10 pounds of tomatoes out of a 15 gallon grow bag in a season, and would, with a bit of effort expect 15 pounds or more. If you grow a heavy yielding variety such as Heintz, expect way more than that.