Laser could be an option in the future to destroy smaller drones, there are already some prototypes.

Otherwise, smaller fast firing, automatic guided guns are suitable - and could fire a lot of inexpensive ammunition.

I.e. there has to be space and enough power to install them.

The Legend class has a only a 57 mm gun and a Phalanx plus several heavy, manually aimed heavy machine guns - the new patrol frigate appears to have RAM instead of Phalanx.