well, i think you could attach a script onto the bow so that it keep your arrow count at a set amount.
The script could look like this:
scn SpecialBowSCRIPT01
Begin GameMode
if (GetItemCount "elvenarrowID" < 20)
player.additem "elvenarrowID" 1
endif
End
this should make it so that anytime the player equips the bow, and the arrow count is below 20, an arrow will be added.
i'm not much of a scripter, so there might be a better way, but this should get the job done
i think OnEquip would work as well, instead of GameMode, but i don't see a difference since the script would be attached to the bow.
goodluck