About 8 of them will kill one of the stone ogres in things betwixt. I had 11 str and 20 dex at the time.

any farming location ? they are more expansive than bolt and magic arrows ...

I invaded a host and his white phantom in the Shaded Ruins. They were understandably cocky, with their heavy armor and untra-greatswords. I started chucking knives at them from a big distance, just kinda taunting them. Then they started shooting with arrows. Well, eventually they came down to me. The phantom was low, like 40%. He died first, after that the host was easy to kill with a few stabs and throwing knives. Thank you, throwing knives. And thank you Hag, for selling them, for the blacksmith will not recognize it as a weapon.

Drops from hollow infantry. It's better to use them on the early areas, because at that point it will take 2 throwing knifes to kill hollow soldiers (forest of falling giants) and 3 to kill royal swordsmans (forest of the falling giants, lost bastille). In later areas, the enemies defense and HP pool became too high and it gets eclipsed by arrows and your melee weapon, so don't be stingy about using it while you can.

Probably the only Souls game where these things are useful.