1. Nowhere does it say you can throw just one. I like the idea of treating them as firearm bullets (as it suggests), and loading the 3 coins of a casting into a shotgun and firing them at a target within 20' as a 3d4+1 ranged touch attack within 20'. Heck, a piece of twine to hold them together allows a slingshot to pack a heck of a punch at close range for that matter.
Nowhere does it say you can violate the standard rules regarding thrown weapons, either.
You can throw one at a time. You get one free attack as part of casting the spell, similar to the free touch attack gained for casting a spell like Chill Touch, but you are throwing only one coin. Later, how many coins you can throw are restricted by the combination of remaining ammunition, how many iterative attacks you're capable of, and whether you are using Two Weapon Fighting.
You can't load them into a shotgun. They aren't firearm ammo. They are similar to firearm bullets, in the senses of the velocity they gain, the damage they deal, hitting touch AC, and being drawn, but not for the purpose of actually loading into a firearm.
Similarly, using them with a sling doesn't help. Their benefits are when thrown, specifically. They're not able to be used as ammunition for any weapon unless you're improvising, and the spell doesn't help you if you do.