hello green7
it all depends


if your growing in a attic there are some thing to consider befor you start
must be able to controle heat above 85 degress dead plants
must be well vented
must be light proof


i would go with 2 400watt remeber mh veg hps flower i would set them up side by side. ok plants you dont have enofe room for most stavia strains so stick with indica or mostly indica hydrids. Number of plants depends on how you wont to grow huge monster plants long ass veg 6-8 plants that gives you a 3by3 ft area for each plant most places bass yeilds off a 3by3ft area scrog id say 10-20 plants sog id say any where from 20-60plants ok yeild huge monster plants aka 6-8 plants 6-8oz dry per plant if from a reliable seed bank scrog aka 10-20 plant 3-5oz dry per plant sog 1-3oz dry per plant but this is just my experiance if anyone wont to corect me feel free hope this helps green7 if you havew any more qestions just ask
chemdawglover
thanks bye