If seedlings befome too crowded in a hydroponic grower, they will compete with each other and not grow very well. Plants can to thinned when they are a few inches tall. The extra plants can be removed and discarded, or, if not damaged, can be transplanted into another grower. As a general rule, when the plants are a few weeks old, the grower can be thinned to keep only as many plants as you believe will fill the container. For a bucket container, this will probably be a few lettuce plants, or one bean plant.
Most seeds have enough nutrient in their seed to last a week or two.
After that they must get their nutrient from the outside environment. When a seedling is 2 to 3 inches tall, it is ready for transplanting into a larger container. There are many types of containers that can be used for growing out plants. Transplanting a seedling can be very stressful on the plant. A seedling, when transplanted into a bigger growing unit, is stressed at first.