i think its usually 3 but im sure 4 would fit not too bushy also maybe a 5gal is too big. im gonna try 3 gallong cloth bags to fit more. if u doing auto 5 is too big, they only veg for like 5 weeks be a waste of soil. my last auto grow even the big ones didn't have a huge root ball. if u did 3 i bet u could fit 4 or 5 especially autos, jusy my opinion but im still learning. good luck
