If you are getting Page 20 of 1, it is possible that the bookmark got created in the wrong place.
What is the last page number in the whole document?
Go to this last page and make sure your cursor is actually in the page. Delete and re-type the last period at the end of the last sentence, just to be sure.
From the Ribbon select 'Insert' and 'Bookmark' from the 'Links' section
In the dialog box that opens enter this name: "LastPage" (don't include the quotes)
Now go to the footer on the last page.
In the footer from the Ribbon select Cross-reference from the Links section
When the dialog box opens select 'Bookmark' in the 'Reference Type:' drop-down.
Can you see the name of the bookmark (LastPage), that you just created ?
If so then select 'Page number' from the other drop-down (Insert reference to:)
Now click on the name 'LastPage'
and finally click Insert, then Close
If your last page was 20 then:
What do you see in the footer.
Page 20 of 20 or something else.