Display page number on custom page title function

You can use this inside your is_page() conditional :

elseif ( is_page() ) { 
 $paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
 echo $page_title . ' ' . $paged . ' of '.$wp_query->max_num_pages;
}

The other way to achieve the same thing. The conditional statement serves to avoid adding a 0 besides the title at the first page :

elseif ( is_page() ) { 
   global $paged;
    if ( $paged != 0 ) {
      $custom_title = $page_title . ' Page Number :  ' . $paged;
      echo $custom_title;
    } else {
      echo $page_title;
    }
 }