Targum zu Bamidbar 7:87

All the bullocks for the burnt offering, twelve, a bullock for a prince of the house of the fathers; twelve rams, because the twelve princes of Ishmael would perish; twelve lambs of the year, because the twelve princes of Persia would perish; and their minchas, that famine might be removed from the world; and twelve kids of the goats for the sin offering, to atone for the sins of the twelve tribes.