With the /plantall command your able to plant more plants even if the planterbox is fully planted.
I tried to plant 99 plants in one planterbox. And the waterconsumption got 11 times higher and i was able to clone all 99 plants in one keystroke.
1700 white berry clones.
If i plant manually one seed at the time, i cant place more than i should. Only when i use the /plantall feature