There are several ways of applying Chapter Numbering. You can apply it as part of a style, or as direct formatting, or type it.
By rights, the Chapter heading should be Heading 1 style, and the user should have used the List Template that shows the word "Chapter" in its picture. If they did, the chapter numbers are part of the heading numbers outline numbered List Template.
Some people elect to type in the chapter number, particularly if the chapters are separate files. If they do this, they frequently forget to check the chapter numbers when they add a chapter. Gotcha!
People working with multiple files sometimes use Heading 1 for the Chapter heading, but manually set its starting number. If you right-click the chapter title, immediately after the chapter number, you can set the Start-At number. This is a fertile source of bother. Any time the user updates the style, they must manually reset all the chapter numbers. If the chapter ever becomes part of a master document, the master document resets the chapter numbering each time a user opens the document. If the template is attached with Automatically Update Styles set, the chapter numbers are reset each time the document opens.
If the chapter numbering is applied by direct formatting, odd corruptions are possible if you redefine the style upon which the numbering is based. The classic case is where the direct formatting List Template is different from the List Template associated with the styles. The cure is to remove all numbering and re-apply just one kind.