What kind of documents do you want to handle? Depending on the type of file uploaded to Alfresco you will need to be able to analyse its internal structure and actually count the pages. There is no such functionality in Alfresco at this time. The tools necessary to do that kind of analysis are also different for different formats, e.g. Microsoft Word vs. Excel vs. PowerPoint vs. PDF vs. LibreOffice etc. etc.
Displaying the number of pages will be quite simple as soon as the information is mapped to a property on the node, but getting that information out of the document will involve quite extensive customisation on the Repository tier if you need to support many different file types.
