Formatting page numbers does not insert them. Do NOT use the command to insert page numbers, though without planning.

Page numbers are normally inserted in headers or footers. If you use the command to insert page numbers at anything other than "current location" it will replace any existing header or footer in the location you specify. If you are editing a header or footer, you can tell Word to insert a page number at the current location. It will insert one, formatted per your specifications.

Automatic page numbers in Word are fields. These can be:

• PAGE
• NUMPAGES
• SECTIONPAGES

They can also be used in calculations.
See How to control the page numbering in a Word document and Sections / Headers and Footers in Microsoft Word 2007-2013.