Rivalry among the Sith resulted in numerous murders among their ranks. They attempted to reduce the problem by working in small groups, eventually operating only in Master/Apprentice pairs. Even so, the entire Sith organization was reduced to just two members by the time of the Clone Wars, both of whom later died at the Battle of Endor.
"Darth" appears to be a title adopted by all Sith when they join the organization. Most continue to maintain a public identity, as well.
Some time between 21 and 40 years after the Battle of Endor, the Sith cult was revived by The Dark Lady of the Sith and her Apprentice, Flint.