Unfortunately, it is not possible to display the total number of images as part of the thumbnail paging text.
As a possible alternative, you could fetch the total number of images in the gallery using the Juicebox-Pro API getImageCount() method. You could then display the information elsewhere on your web page alongside your gallery.
For example:
<script src="jbcore/juicebox.js"></script>
<script>
    var jb = new juicebox({
        containerId: 'juicebox-container',
        galleryWidth: '800',
        galleryHeight: '600'
    });
    jb.onInitComplete = function() {
        var total = jb.getImageCount();
        $('#count').html('<p>Total Image Count: ' + total + '</p>');
    };
</script>
<div id="juicebox-container"></div>
<div id="count"></div>