| Cubali |
Yeah, to poison a quiver full of 20 arrows, you'd need 20 doses of poison.
Whether or not you think you should be able to squeeze out more uses of a vial of poison when you're only poisoning arrowheads as opposed to a greatsword is up to you as a house rule, but that opens up an IMMENSELY complex can of worms.
Does it take the same amount of poison to coat a human's greatsword as it does a halfling's greatsword? What about a pixie's arrows as opposed to a storm giant's arrows? Does it take longer to poison a tiny arrowhead than it does a greatsword? If it takes longer, because you have to coat more surface area, and thus uses up more poison, do you have to worry about poisoning yourself accidentally more often? How much do variant sizes of poison cost? What do different size vials weigh?
Usually when you see a rule that's simplified such as the poison application rules, it's for a reason.
The Weapon Blanch items work in this way without any real problem, though; "Coating 1 weapon or up to 10 pieces of ammunition" and the vocabulary used in describing it, (e.g. dose, remaining effective until you make a successful attack), are the same as with poison. I would think that the number of times you would be exposing yourself to accidental self-poisoning, through treating each piece of ammunition and the possibility of rolling a 1 on the attack roll would be the natural counterbalance for the effectiveness of poisons.
And that doesn't even consider the higher risk of wasting the poison involved with projectiles, through unrecovered(lost) arrows from missed shots, (where as the poison isn't expended on a melee weapon until it scores a hit.) Each arrow that misses its target, yet isn't recovered at the end of the battle, due to the 50% chance of being lost or damaged, is essentially 1/10 of your poison expended without scoring a hit, assuming you were using the Weapon Blanch rules for poison. If you weren't its a full vial of poison wasted.