2d4 force damage.
You can choose two targets; roll half the damage dice for one missile and half the damage dice for the other, then assign one set of damage dice to each of the two targets.
Roll a d20 when you use the spell; if you roll a natural 20, the magic missile crits and deals double damage. (Rolling a 1 is not a fumble; this roll checks only to see if you can crit.)
The 7th and 9th level versions of the spell now use d8s as damage dice.