It is really not about the number of plants you have but space you have...this thread was done with the small operation in mind....like the example 9 square feet....equals 9 plants if done right....the majority of grows are small cabs and closets and I would imagine that a 400w puts out a heafty amount of heat and this small area would be hard to control heat.....the other reason for 2 is the height factor between plants....I use 2 because I stagger harvests and not all plants are the same height with 2 I can have 1 on taller level and the other on the smaller babes....cant do that with just one light...also I am a big advocate of side lighting as this seems to keep the stretch level down somewhat and bushes the plant and lower buds much more...I have a combined 8000 lumens on back side with the T8 floros...with that being said I do not need the penatration benefit of larger wattage lights...but you are correct more light equals more plants if you have the space.

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(Genesis 1:29-31):Then God said, I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food. And to all the beasts of the earth and all the birds of the air and all the creatures that move on the ground-- everything that has the breath of life in it-- I give every green plant for food. And it was so.