Firing Missiles

New to the game and was wondering about the rules regarding the missiles fired by the flying units. Essentially I'm trying to find out if the flyers can fire their missiles at multiple targets or if only one unit can be targeted each turn

1 target unless it's a superheavy flier.

ALL units in the game can only fire at one target per turn.
The only exception to this is the Split Fire special rule which some units have, but sadly no Dark Eldar units have.
(All Super-Heavy Vehicles and Gargantuan Creatures have it automatically).

To be clear, superheavies and gargantuans DON'T have split fire. They have a different special rule that applies to them that allows them to fire each of their weapons at whatever number of targets they like.

Split fire just allows a model to fire at a different target than the rest of their unit.