Check the page: http://darksoulswiki.wikispaces.com/Dark+Hand
If you know of others NPCs, please tell me which one and how much humanity you can drain from them. Remember that raising your rank in DarkWraiths only lets you drain more humanity per try, but the total humanity you can drain from NPCs remains the same.
This is to make people's journey through draining humanity easier, so they don't have to check ALL NPCs pages, they can just see the list and see who they already drained from and who they didn't do it yet. How much they can get, etc.
A nice detail: You can drain a total of 22 humanity from Petrus, Reah, Nico and Vincent. So, when you see all of them at Firelink, Soulsuck... I mean drain their humanity! It's free, and doesn't make them hostile.