Well, majority of stuff is seen by algorithms, until something triggers the flag, no human sees it, mostly due to amount of data that needs to be checked.
As for how many quit due to what they have seen... Well, they do deal with killers that do weird shit with dead bodies, pedophiles and other sick fucks. Most normal people would probably lose faith in humanity after a month of such work and hanged themselves.
Wew, this just went dark. You are welcome!