Put it this way, my plants are each 16 inches tall, and over 24 inches wide, do the math and you'll know the answer. You said your box is 21 inches long. Your fan leaves and branches will be jamming up against the other plants main stalk, and thats if you put only 2 as far away from each other as you possibly can.

I started from seed in 5.5" netpots in 1 container and left them in there. Mistake
imp: