Hi Omar,
Assuming that you are using the aforementioned "Textbook" stylesheet, you made the modification outlined in my original comment, and the first cell in the Notebook is a "BookChapterNumber"-Cell, replace the first cell with following Cell expression:
Cell[TextData[
CounterBox["BookChapterNumber"]], "BookChapterNumber",
CounterAssignments->{ParentList, {"BookChapterNumber", 1}}]
I assume that you used the Textbook styled-notebook template as the basis for your document layout. FYI, the template can be accessed from a palette that is summoned via the menu item, File > New > Styled Notebook.... (see attached screenshot of the palette) In the palette, look for the icon with the "Textbook" caption (i.e., middle row, left-most column) and click the upper half of the icon. Note, when you hover over the top-half of the icon, the word "New" will appear. Click the icon to open a new template.
The addition of the option, CounterAssignments->{ParentList, {"BookChapterNumber", 1}}, should be the only difference between the original templated "BookChapterNumber"-Cell and the one above. (Important: The option's rhs value contains the sublist {"BookChapterNumber", 1}. That designation assigns the value 1 to the "BookChapterNumber" counter. However, the value is immediately incremented by 1 because the appearance of the "BookChapterNumber"-Cell in the Notebook causes the counter to increment. CounterBox["BookChapterNumber"] will display 2. Generally, the value assigned to "BookChapterNumber" must equal the desired value - 1.
If each chapter is in it's own separate Notebook, replace the "BookChapterNumber"-Cell with the one above, but be sure to replace the value 1 with a value == desired value - 1. For example, for chapter 3 replace the 1 with 2,..., for chapter 10 replace the 1 with 9. Also, remember to add the StyleData["EquationNumbered"]-Cell, as described in my original comment, to each notebook to correct the equation numbers.
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