Found in the back of Tsubasa: Reservoir Chronicle Volume 1.

"The first volume of Cardcaptor Sakura was released in Japan in 1996, and by the time the series was finished it would number twelve volumes in all. The first six, called simply Cardcaptor Sakura, are the story of fourth-grader Sakura, who finds a magic book called The Clow in her father's library. The book and its magical guardian, Kero, were responsible for containing the Clow Cards. The Clow Cards, created by sorcerer Clow Reed, have escaped, and Kero needs Sakura's help to find them. Empowered with the key to recapture them, Sakura becomes a Cardcaptor and begins her quest for all of the lost Clow Cards.

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In the manga, the main storyline ends in volume six when Sakura successfully captures all of the Clow Cards and is named Master of the Clow. It almost looks like it's time for Sakura to hang up her wand for good, but CLAMP still had a few more stories in mind– and another six volumes to go!"