It depends on how much room you have? you can do 6-10 plants under a 250 if you keep them close and keep the area of what your trying to do small. if you are planning BIG yeilding plants you will at minimum need a 600. Its all in what your looking to do. More light always yeilds more then small lights. so let that be your light selecting guide. Remember there is other factors then how many plants that determine yeild. 4 plants topped and groomed under a 250hps and you will definately be able to grow enough for yourself and a few friends if you know how.