Obviously it depends, but i'd suggest a gallon per foot of height and 2/2 space. If you intend 2 foot plants to cover a 2/2 space each do 2 gallons(a foot of soil in the buckets not counted). If you intend for them to be 5 feet and cover a 2x2 space do 5 gallons. If you intend for them to be 3...