Actually, giving your pages a number in Word is very easy and probably you know how to do it. In this case, I figured out that one of my documents where with a bad structure, just see the image and you will notice that the total of pages is incorrect because the document has more than one page.
To modify this situation it's necessary to edit the field which is responsible of showing the total of pages, you have to situate over the total and use the right click to show the menu, where you'll have to select "Edit Field...".
This option will show a dialog where we are going to reset this counter by using the field "NumPages" and he can set some properties depending on our likes.
And this is how the counter will show the reality of the document.