The series main story will end at chapter 12, the last chapter will be an extra. The extras for volume 1 are on Chapter 6.5
I updated the chapter name so people don't miss that 6.5 is the extras.
also the clear up any confusion, on the official English website the extra chapter is always counted in the chapter listing as a number and not as an extra, MangaPlaza always does this for their manga. So on the official website the extra for volume 1 is counted as chapter 7, making the actual chapter 7 appear in the list as chapter 8. the series finial chapter (chapter 14) will be an Extra. Hope this explanation helps.
Wow! The chapters increased! I thought it would end at chapter 12 since there where no extra chapter last time