CK Note: Word 2007 - 2013 interface has an different automatic numbering scheme which I have been told is much less subject to corruption. Microsoft Word 2010 Bible by Herb Tyson, MVP. However numbering is still very imperfect in these later versions. I still recommend following Shauna Kelly's step-by-step instructions (see above) if setting up numbering in a template or in a document likely to be heavily edited. If you start without doing this and end up with "spaghetti numbering," fixing it will be a very large chore!
A best practice that we recommend to our clients is to create a base/folio master –with styled and positioned footers and current page number special characters– on which all other masters are based. This allows a footer that may contain date or issue information to be updated once and the changes are reflected in all of the master pages. If your masters only have current page number special characters then you can just add them to each master and the page numbers will be reflected when each master is applied to your document pages. document numbering in indesign