Well, it was obvious from the intro that they were providing this chapter sheet by sheet with bd releases. I don't know how it can be understood the wrong way.
Sheets with only 1 page on them and blank backside? How do you imagine to read such manga when you assemble it?
12 sheets? Well, we can count number of pages right here, from the chapter. It's 14 here, so, I guess, they gave out a cover with first and last page, so that they are fixed. That gives us 14 pages and 6 sheets to insert frivolously between start and end