Painting a 12 x 12 room with eight-foot tall walls would need 1.5 gallons of paint. The average paint coverage per gallon is approximately 400 square feet for one coat. You need adequate paint for your room to avoid headaches because you don't have enough, since you need at least two coats to completely cover the surface of the wall. Four-gallon paint cans cover up to 1600 square feet, which is enough to cover several average rooms.
You can consider this amount if you have a great room or another very large room.