In my adventure I want to add some winged kobolds, but I don't know how their "Dropped Rock" attack works.
Do they carry a pouch of rocks, or they need to pick up larger rocks from the ground every other round?
From the intro to the Monster Manual (p. 11), under "Ammunition":
A monster carries enough ammunition to make its ranged attacks. You can assume that a monster has 2d4 pieces of ammunition for a thrown weapon attack, and 2d10 pieces of ammunition for a projectile weapon such as a bow or crossbow.
The Dropped Rock attack of the winged kobold would be a thrown weapon attack, so it could be safely assumed that it carries 2d4 stones.
If you decide that a winged kobold can only carry the weight of one rock in flight, you can rule that they need to land and pick up a new rock between attacks. This may make them a little easier to defeat, as landing will bring the kobolds into range of the party's melee fighters.