There is no best size for ebb and flow. It depends on how much light you have and the area you are trying to cover. In a 4 ft by 4ft area, running a 1000w light, you can go from anywhere from 10 to 30 plants. A lot depends on how big they are when they go into flowering. The smaller the plant at flowering, the more room you have for more plants.

Please remember they can more than double in size by harvest time