I have a question about 400 hps lights. I have 12 clones growing with CPls and had a doubt about how much wattage i need to put those 12 clones to flower, beacuse im not sure if 400 watt hps light will be sufficient. the plants are in space which is 3.5ft wide and 3ft long and they are in 4 rows, 3 plants in each row.. ty very much, please write back.

i think 400 watter and you will be just fine

A good place to aim is 50w/sq ft, you have 10.5sq ft and 400w = 38.1w/sq ft so your not far off. Your plants will grow ok with that light, if you already have a 400w. If you are looking at buying a light I would suggest a 600w. That would give you 57.1w/sq ft, much better!
Whichever sized light you use, look into cooling it somehow, with a DIY cooltube made from a bake a round if your on a budget or a proper air cooled hood if you're feeling flush. Doing this would allow the light to be closer to the tops of the plants, giving them more lumens and growing fatter buds, at the same time as making the most of whatever height you have in your room.

I have a 400 in a 3x4 space. It works well. But you can tell the difference between my buds and those grown under a 1000 watter. Those buds are much more dense. A 600 is really a good compromise, but more watts mean more heat. In a small space like 3x4 or 3x3.5 you will need a good fan on your light. I have a 171cfm on my 400 in a breadtube,cooltube. It works very well, exepct in the summer, there should be a 250 on it. That being said If you get a 600, make sure you get a fan that will move enough air to keep it cool. Of course the same goes with the 400, you just dont need as big a fan.