WP_Query() not return all posts

I have i WP query: $flag_query = new WP_Query('post_parent=6&post_type=page&order=desc');

i have 13 pages that shoud be returned but i can only see 10 of them

here is my code

$flag_query = new WP_Query('post_parent=6&post_type=page&order=desc');
                        ?>
                    <ul class="network-participants">

                        <?php while ($flag_query->have_posts()) : $flag_query->the_post(); ?>
                          <!--  <li><?php the_title();?></li>-->
                         <?php    if ( has_post_thumbnail() ) {?>
                         <li><a href="<?php echo get_permalink(); ?>">
                              	<?php the_post_thumbnail(); ?>
                             </a>
                         </li>
                        <?php }
                        endwhile; ?>
                    </ul>
					<?php wp_reset_postdata(); ?>

any ideas?