Word produces page numbering with a simple built-in field that it automatically updates as it paginates. Page numbering is very stable and rarely gives trouble.
It is worth mentioning that in Word, "pages" do not exist in the document file. Like a professional typesetter, Word makes up its pages on the fly when it displays or prints a document. Word uses measurements from the installed fonts and the installed printer driver to do this. It is almost impossible to get two machines so exactly similar that a document will paginate with exactly the same page breaks on each. Sometimes people complain that when they open the document on a different machine, some of the page numbers in the TOC or Index are "wrong". They're not: when the document is opened on the other machine, minute variations in set-up that do not show over a ten page memo will cause variations in the position of page breaks in a 1,000-page manual. If you remember to update the TOC and Index before you print, the problem corrects itself.
The other classic cause of bad page numbering is multiple "sets" of page numbers in the document. There are two places you can add page numbers: to the "document", or to the running Footers. If you use Insert>Page Numbers, you add the page numbers in little floating fields that sit in the drawing layer above the text layer. If you open View>Headers and Footers and click in a footer, then click the hash sign # you insert the page number field in the footer, on a section-by-section basis.
Unfortunately, there's nothing to stop you adding them in both places, and having them start at different numbers. If you find that the numbering is out for just one section of the document, this is a classic sign that you have conflicting sets of page numbers in the document.