Count each kill as one used up arrow.
I scoured the rulebook and couldn't find any rules for it. It seems that since tracking arrows is an optional rule anyway, they didn't feel the need to specify.
However, I did find a comment from author Andrea Sfiligoi in this Facebook thread:
Multiple attacks. If you want to limit it, you can keep track of the arrows and count each kill as one used up arrow (realistically, most arrows may not be recovered from the targets).
For context, the discussion is about the realism of killing multiple enemies with one arrow attack. It seems to be the author's stance that each attack is an abstraction that might actually be several swings of a sword or shots from a bow. So, if you care about the granularity of tracking arrows, have a hit or miss against one enemy use one arrow, and hits against multiple enemies use one arrow each.