How can you increase ammo/arrows for NPC?

I checked the inventory and so far I have to keep dragging and dropping the custom arrows I made for my NPC into his inventory. I have 100 all in one long list but surely there's a way to alter the count so it would just say :CustomArrows, Quanity/Count: 100. Instead of just singular arrows going all the way down the list. It looks messy too in his inventory or when he gets looted after dying in combat.

Any help appreciated.

A slow 2 click on the item count will allow that field to be modified, and a higher count to be punched in.
Only one instance of an item is normally needed.

Or press F2

Don't quote me on this, but I'm pretty sure that as long as an NPC has at least one arrow in their inventory, they'll have unlimited.

There's two separate issues here:

The number in inventory can be set so you don't have to add them one at a time, but the number provided does not limit how many they can use!

IIRC an NPC can use any ammo type they are given, but those do not get used up, so when you loot the corpse they'll still have the full set they started with. That's why giving them one = unlimited as far as being able to fire them at you is concerned, but if you want to loot ten from their corpse, you'll have to give them ten!