How many plants under a 400W MH?

Hi I have a 4x4x6.5 tent with a 400w MH and was wondering what the maximum number of plants I can grow under this light, is it just how ever many I can fit in the tent?

Pretty much. You'll have to take into account the particular strain that you are working with to decide how many to plant initially. I would say around 6-12 plants in a 4x4 area, depending on how much the plants are expected to grow during flowering. Most indica plants bush out more but stay shorter, allowing for less plants per square foot, and less of an issue with height. Sativa varieties tend to stretch considerably more and are generally slimmer than indicas. 12 or more thin sativas could be packed into the space, but they will grow taller, sometimes tripling or quadrupling their height during flowering.

36 or 1 or anything in between, based on veg time and a 3x3 footprint.