According to Weekly Shounen Magazine, CLAMP's manga Tsubasa RESERVoir CHRoNiCLE
will end in WSM volume 44 to be released on September 30th. It will be chapter 232. The final tankobon volume 28 will be released on November 17th along with CLAMP's art book.
Update on September 19: The official website of CLAMP
has announced on September 16 that the final chapter of Tsubasa RESERVoir CHRoNiCLE will be published in the 45th issue of Kodansha's Weekly Shonen Magazine on October 7, not in the 44th issue as previously stated.
Scan of WSM