Mirror images have the exact same AC and and AC type as the caster.((This was changed in later editions of the game, but in AD&D,

For targeting purposes, since the mirror images are exactly mimicking the caster, they do not count as separate targets for target determination. The caster remains but one possible target, and if the caster is struck, then a die is rolled to determine if the caster or one of the caster's images was actually struck.