One seed obviously grows one plant. That being said, it varies on strain, size care, and many other factors. I regualarly have outdoor plants that are 7ft tall and produce around 1/2lb each, I have also had plants that were smaller and only produced 1oz. Indoor plants can even be re-vegged and have a second or third harvest, which doubles or tripples the harvest. Then if you get into cloning one seed can produce pound upon pound for years even.