^ It's hard to not notice it when Yuuko herself pointed it out in the last chapter...
As far as I've understood, the 28 volumes thing was never confirmed by CLAMP themselves. Appearantly there was an interview where they said that they had finished 2/3 of the story (or something like that), fans looked up at what chapter they were working on when the interview took place, and, since we know the average lenght of a TRC volume, they came to the conclusion that the finished result should land at around 28 volumes.
I wouldn't worry, though. There are still a bundle of loose threads that need to be explained. The last volume does not necessarilly have to end with the final battle - it could be dedicated to flashbacks and explanations. Most of RG Veda's final volume was like that, for example.