add_filter('the_title', 'generate_category_post_count_title', 10, 2);
function generate_category_post_count_title($title, $post_ID)
{
    if( 'nav_menu_item' == get_post_type($post_ID) )
    {
        if( 'taxonomy' == get_post_meta($post_ID, '_menu_item_type', true) && 'category' == get_post_meta($post_ID, '_menu_item_object', true) )
        {
            $category = get_category( get_post_meta($post_ID, '_menu_item_object_id', true) );
            $title .= sprintf(' (%d)', $category->count);
        }
    }
    return $title;
}