well roughly 6 sq. ft can be effectively lit with a 400 watt HPS to sustain a rate of 70w per sq.ft. ..... so however many you think you can get in there n have ROOM to grow...... id have to guess 5
im not familiar with SOG (never done it) but can say LST n u'd have room for 5-6 plants if you tied e right.
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For optimum bud density, consider covering no more than 4 square feet--assuming you are using a sog variety like northern lights, black domina, etc. Use reflective walls to maximize light effectiveness--flat white is fine. True sea of green=4 plants per sq ft or 16 plants. Anywhere from 4 to sixteen plants--I recommend 8.