By the
RAW for craft, I think a character can make 20 arrows in about half a day, assuming a Craft DC 12 (since that's the DC for bows and simple weapons). I'd assume several craft skills are applicable: fletching, bowmaking, weaponsmithing, woodworking, etc. If out in the wild, then apply a -2 circumstance penalty to the roll for using improvised tools, and
[houserule] maybe a -2 for improvised materials. Also
[houserule] rangers and archers (ie, characters with proficiency with bows) might get competence bonus equal to level, too, since that seems to be a reasonable part of their class knowledge.
Don't forget that a PC can recover an arrow 50% of the time if he misses his target.
I'd think a PC could certainly keep at least 20 spare arrows in his pack, in addition to the one he uses actively from his quiver (I don't know how many quivers he could be wearing, though). The only real limit is weight: 20 arrows weigh 3lb.