My personal experience doing this for two characters on Mangler TLP server is that it's fairly rare. There are approximately 10 restless souls that spawn at the camp and for the first character I did the quest on, I cleared the camp probably 5-6 times to get 4. Didn't seem all that rare. I'm now doing it on a second character and have cleared the camp 5-6 times and gotten 1.
Edited, Jul 30th 2020 11:22am by rahzel44