I roll them per missile. However, making it one roll for all missiles makes those abilities that add a stat bonus to one die roll more powerful when used with magic missile.
Though one can, of course, just add the stat bonus to each die roll. The effect is the same even if it's not strictly correct by the rules.
Though one can, of course, just add the stat bonus to each die roll. The effect is the same even if it's not strictly correct by the rules.