Even a single fixture will give good light for the GH, but that's basically light for us humans, not plants. The further away from the light source, the less usable lumens for your plants. It would help to know what you are growing. Do you have shelving on both sides of your GH? I have two, 10'x12' GHs, and have four fixtures in each that are 6500K, four-tube fixtures. These are just typical, inexpensive shop lights bought at Lowe's. The fixtures are set to come on at 3:00 PM and go off at 9:00 PM. Those fixtures are within 1/2' of plants on the top shelves but are 2' from the middle shelves. I have supplemental LED lighting for plants on the lower shelves, but I am growing small orchids on the lower shelves. Larger orchids are on those top two shelves.
I have just ordered two, 4', six tube fixtures that use T5H (Gro-Light) tubes rather than typical florescent tubes. These tubes produce almost twice the lumens as my T5 florescent fixtures. There will be a mix of (3)6500K and (1) 3000K tubes. Will these be any better than those 4', 4-tube fluorescent fixtures? I don't know but might have an answer by spring, 2015. By the way, 6500K is best for growing plants and germinating seeds/seedling, and the 3000K is best to stimulate flowering.
