How many watts of light per plant is sufficient?

I have 800 watts for 5 plants. That breaks down to 160 watts per plant. Right now I have 1x 400 watt HPS and 1x 400 watt MH conversion bulb. I'll use the other HPS bulb when I go into budding.

So is that enough light for 5 plants or should I keep it to four?

Its good for 6 plants i use 2 hps 400w and get about 140 grams perplant with 4 weeks veg

you have just the right amount of light for five plants in flowering. 800 watts hps is about 120,000 lumens. if you assume about 3 square feet per plant then you have 8,000 lumens per square foot. the recommended amount of light for flowering is 7,000 to 10,000 lumens per square foot.