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.