hughbert,
current.datapage will return the current page, but not the total amount of pages.
To return the total amount of pages, the best way is to use the Metadata feature. In your Document structure, right-click on the Metadata folder, and choose "Refresh Metadata". Then make a new data selection, and at the top of the data selector, click on the Metadata tab.
There you'll have a list of metadata tags to choose from. You have one called "PageCount", which is the total number of pages. You also have "DatapageCount", which is the number of data pages. Highlight the tag you want, and click OK, and that should make a data selection of the metadata tag.
Hope that helps.
Regards,
Rapha