You get every single believer listed. Every first, second, third, etc. level believer is under the Thrallherd's command. And no, you can't use Leadership and the Thrallherd extraordinary at once; don't get broken up about it though, the Leadership feat is a terrible attempt to introduce follower mechanics anyway. I banned it from my games ages ago.
alright thank you for that clarification! anyone else have any suggestions or comments?