Complex sentence: Immensely successful, it ran for nine seasons--television's third longest running western, behind "Bonanza" at fourteen seasons and 430 episodes, and "Gunsmoke" at twenty seasons and 635 episodes.
Split into simpler sentences: Immensely successful, it ran for nine seasons--television's third longest running western. It follows "Bonanza" at fourteen seasons and 430 episodes, and "Gunsmoke" at twenty seasons and 635 episodes.