To increase the number of Elf-shot arrows for your bow that you have available in Middle Earth: Shadow of War you have to find better bows. The number of Elf-shot that you get is specified by the equipped bow. Due to the Shadow Strike skill, which is a powerful skill, but it requires Elf-shot, I recommend preferring the number of Elf-shot over the damage of a bow.

The Best Skills and Upgrades
All Shadow of War Guides
All Combat Guides