each 1000W lamp could cover 4x4 feet without a problem, so 32 square feet of coverage. it's up to you how many plants you want to fit in there. for the most part i don't think there's too much benefit from going any denser than one plant per square foot, so you're looking at i'd say anywhere between 4-32 plants.
remember that fewer plants are easier to maintain, but more plants will take less time to fill the space. you also get more efficient canopy coverage with more plants.