Eighteen is considered a special number in Jewish culture because it is the numerical value of the Hebrew word chai, meaning “life.” In Hebrew, each letter has a numerical equivalent. For example, the first letter of the Hebrew alphabet, aleph, is equal to one. The second letter, bet, has the value of two, and so on until we reach 10, 20…100, 200 and so on. The Hebrew word chai comprises the letter chet, which has a numerical value of eight, and the letter yud, which has a value of 10.
As a result, it is common for Jews to give gifts in multiples of 18, especially for a Jewish lifecycle event such as a bar or bat mitzvah or a wedding, or when donating to charitable organizations, especially to Jewish charities.
Moses spoke to the heads of the Israelite tribes, saying: "This is what the Eternal has commanded: If a householder makes a vow to the Eternal or takes an oath imposing an obligation on himself, he shall not break his pledge; he must carry out all that has crossed his lips." - Numbers 30:2-3