| Fuzzy-Wuzzy |
For most purposes 50 pieces of ammo act like one weapon, but bless weapon specifies "Individual arrows or bolts can be transmuted, but affected projectile weapons (such as bows) don't confer the benefit to the projectiles they shoot." I don't see that making the spell into an oil should make it vastly more efficient, so I'd say it applies to one piece of ammo, like poison.