It depends, are these auto's known for really fast finishing and staying more to the small side.. if so then they can actually do quite well in 2gal pots so by that ratio you'd be fine to plant them all. The trick is watering. Don't try to soak the entire 15 gal. When they are 1st starting just water the area right around the plant really good and never let it dry all the way out until established, then a dry spell is ok. As they get larger start watering further away from the plant so the roots search for water, if they droop a bit then hit them closer.. I water every other watering near plant, away from plant, near plant , away from plant.. me little secret of many.. luck
