Lamps or candles symbolize wisdom, eliminating the darkness of ignorance. In Tibetan monasteries hundreds of lamps are lit as offerings. There is really no limit to the quantity of either water bowls or lamps. Visualize that the offerings are blessed. Whether external offerings become pure or not, or whether they become a cause for good rebirth in the next life, a cause to achieve liberation, or a cause to achieve enlightenment to benefit all beings depends on one's motivations and dedication.
Dedication is crucial.
It will not exhaust or limit one's store of merit but will multiply and increase it. It is excellent to dedicate the merit of making offerings to the elimination of suffering and its causes from all beings, to their achievement of lasting happiness, and to world peace. At the end f the day, before or at sunset, empty the bowls one by one, dry them with a clean cloth and stack them upside down or put them away. Never leave empty bowls right side up on the altar. The water is not simply thrown away but offered to the plants in your house or in the garden.