If you're growing in a 20 gallon planter it be ok, but you're not and the planter would have to be 4ft long so the plants don't crowd each other. I've seen several plants grown in 4x4 planters or 100 gallon smart pots.
Size-size-size--- Its all about the size of the pot and at least 8"-12" apart
20 gal Quare pot- 1-lady in each corner- water the center and root will chase the water..