Rashi on Exodus 15:27/רש"י על שמות ט״ו:כ״ז

1שתים עשרה עינת מים TWELVE SPRINGS OF WATER — A number corresponding to the twelve tribes of Israel were ready for them (Mekhilta).

2ושבעים תמרים AND SEVENTY PALM-TREES — corresponding to the seventy elders (Mekhilta).