There are up to 15 slots in Herbert that can be used to grow plants. The spacing inbetween the plants was chosen that the plants all receive enough light. We recommend to leave one slot free and cover it with one intransparent cap. Use that free slot to refill Herbert when the water tank is empty.In one slot you can have more seeds depending on which plant you want to grow. You can find a recommendation of how many seeds to put on one sponge on each seed package