From Bulbapedia, the community-driven Pokémon encyclopedia.
A chapter is the Pokémon Adventures equivalent of the anime's seasons. They divide the manga into sections, which have a separate plot, though the characters and events from one chapter may be used in another, later chapter. Each chapter is named after a main series game, and the characters introduced in each chapter generally share those names.
Chapters are further broken down into volumes, which are then separated into rounds. There is no definitive number of rounds in each chapter.
Ten chapters have been released, of which first eight have been fully translated into English and published by Chuang Yi. Some rounds of the HeartGold & SoulSilver chapter have been released but this is not yet complete.
VIZ Media has fully translated and published the first three chapters and is now, after a publishing break, translating the Ruby & Sapphire chapter. In addition, the company is currently translating Diamond & Pearl chapter and Black & White chapter as a second and third series, and is planning to release the HeartGold & SoulSilver chapter as a fourth series starting in August 2013.
List of chapters