According to the Greek mythology, how many rivers were found in Hades?

In Greek mythology, Styx is a deity and a river that formed the boundary between Earth and the Underworld (the domain often called Hades, which also is the name of its ruler). There are five rivers in Hades: Styx, Phlegethon, Acheron, Lethe, and Cocytus. All these rivers converge at the center of the underworld on a great marsh, which sometimes, also is called the Styx.