I don't dare to reproduce it yet, but I'm certain I had 21 arrows before sparring. I shot someone twice point blank, during a spar by mistake.
When I opened my quiver again, I had 13 arrows left, having consumed 4 arrows per shot. I had 8 copper arrows, 8 stone arrows, and 5 stone arrows in separate stacks.

Hopefully this helps!

*Edit
Maybe it consumes arrows if you try to fire before the cooldown is done? Also I've lost like, 30-40 arrows to bugs now, I'm done testing it I think.