wp_link_pages: display current page number only if has multiple page breaks?

The $multipage is constructed inside the setup_postdata() method of WP_Query.
It’s called when you run the_post(). So just as @s_ha_dum explained, you have to make a loop around it:

You can try this:

if( have_posts() )
{ 
    the_post();                      // Make a call to setup_postdata().
    echo sbwp_get_current_chapter(); // Display the current chapter.
    rewind_posts();                  // Rewind back.
}

before your main loop, where you must remember to rewind back.