See:
http://kodi.wiki/view/Skinning_Manual#Window_2
Code:
$INFO[Window(home).Property(Movies.Count)]
This will show the total number of movies scraped to your library. It will work anywhere in the skin except inside a list itemlayout/focusedlayout container iirc because window properties cannot be accessed inside lists.
You can replace movies.count with any of the following properties:
Movies.Count, Movies.Watched, Movies.UnWatched, TVShows.Count, TVShows.Watched, TVShows.UnWatched, Episodes.Count, Episodes.Watched, Episodes.UnWatched, MusicVideos.Count, MusicVideos.Watched, MusicVideos.UnWatched, Music.SongsCount, Music.AlbumsCount, Music.ArtistsCount