I chose 40 for one reason, range and numbers.
Yes, with the average 50 HP unit assuming 15+ damege each, 4 arrows is enough to kill them. This means 6 kills each, and thats asusming perfect accuracy and alot of stats.
In reality, getting jsut 2-3 kills on the battlefield with archery without at least 5-6 power drawand/or horse archery, I end up using all 40 just getting a few kills, and thats just not ok for me.
The problem comes when you face 100 man armies, even 50-60 armies, you on your own cant do much (thanks to the battle system), enemy throwing weapons only take 2-3 hits to take you down, and with large numbers, your done, so standing as melee cavalry after making 6-10 kills(assuming perfect shots).
The solution to this SHOULD be troops, right? no. Vagris is the only faciton with truely frightening archery, and my experience with crossbows is well, 8-15 damage WITH the same proficiency(about 150-300 by mid/late game), so swabian and rodocks archery is at best, support, ad thats assuming the infantry line holds.
So nearly double the amount of arrows, is very big effect, especially if your going for pure archery.
Any argument otherwise would be nice