Pages for iCloud allows you to create and edit your documents online. Pages for iCloud has the same document setup options as Pages for Mac. Adding page numbers and turning Facing Pages on in Pages for iCloud are slightly different, however.

Create a Pages or Numbers template, or a Keynote theme. When you add it to the Template Chooser or Theme Chooser, it appears on all of your devices that are signed in to iCloud with the same Apple ID and use iCloud Drive with Pages, Numbers, and Keynote.*

Add page numbers in Pages on Mac. You can add page numbers to your document that automatically adjust as pages are added or deleted. You can add a page number to a header or footer on your page, or elsewhere in the document–for example, within body text