3 lugs fits nicely in a 20gal so......... I agree 5 should work nicely in a 32gal but a 6th may or may not fit especially if you have a high brix must and have to water it back.
If I understand that a lug is a 36lbs box of grapes then 6 should yield you about 21 gallons.... Cutting it close but doable... 2/3 full about as much you want to go and be able to punch down....