There are a total of 45 unique Bone Charms in Dishonored, but far fewer of them are found in any given play though. This is because each charm found is randomly selected from the complete list. What stinks though, is that Bone Charms usefulness varies wildly. Some, like the .5 second chokehold are massive boosts. Others, like Albino, seem to be totally useless.
If I were an... ahem-Dishonorable gamer, and interested in savescumming for better Bone Charms, from what point would I have to reload? Is it determined at Mission Start, at entry to the 'zone' where the Charm is located, when it's picked up, or at some other point?