I believe i have seen somewhere around sugestion that special arrows should either be much cheaper, or be sold by stack of 10.
If that would be the case, personaly i would probably never again shooted regular arrow, so ... who gives a damn?
Also, im not quite sure how would you like to manage this ...
I wonder what count of arrows would be apropriate ... 20 seem too little to any serious encounter, but on the other hand if you quadruple it, since every of your characters can drag his own stack of arrows, you have 80, wich on the contrary seem like a little too much, so we are back in "practicaly unlimited". :-/
Also, if you wish to limit arrows just by their weight, i believe our characters should be able to drag around 200 arrows (read as: 10 quivers) each, concidering they dont want to have anything else with them at the time ... and since you can simply "send all loot to camp" there is even no problem with that. So once aggain, practicaly unlimited. :-/