There are never more than two Sith lords at a time - one master and one apprentice. The given reason for this is that when the Sith order first came into existence, there were many Sith, but consumed by greed and desire for power, they began destroying each other until only one was left - Darth Bane. To guard against this happening again Darth Bane only took one apprentice and started the one-master-one-apprentice tradition.
When Palpatine and Darth Vader attempt to corrupt Luke Skywalker, each have their own plans for the future. Vader desired to kill the Emperor and rule the galaxy with his son, but Palpatine wished to replace Vader with the boy. Neither goal was successful, and it appeared that the Sith had finally come to an end when the Death Star exploded.
According to the Expanded Universe, however, other Jedi sank into Darkness later. Even Han Solo's daughter Jaina, overcome by grief at the seeming loss of both of her brothers, touched the Dark Side of the Force.