I would do something like 6 plants since a 400 watt HPS doesn't have very much light penetration I would trim lower branches to get to get the upper buds as much energy as possible. I use to two 400's in my tent and I trim a good amount off the lower portion of the plants. If your growing in soil I would go into flower no earlier then a month. Now that I'm using hydro I can flower at about a month and use a good amount of bushmaster to keepem tamed. 6 is def your number though for how many.:thumbsup: If doing SOG.