Why does posts_per_page doesn't work in Wordpress?

I'm trying to take 12 posts per a page, but it displays only 4 posts.

Has anyone encountered this problem?

Current Code

$paged = get_query_var('paged') ? get_query_var('paged') : 1;
$args = array(
    'post_type' => 'post',
    'posts_per_page' => 12,
    'order' => 'DESC',
    'paged' => $paged,
);
$lists = new WP_Query($args);
echo $lists->post_count; // this outputs 12!

<ul>
    <?php if ($lists->have_posts()) : ?>
        <?php while ($lists->have_posts()) : $lists->the_post(); ?>
            <li>
                <div>
                    <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
                </div>
            </li>
        <?php endwhile; ?>
    <?php endif;
    wp_reset_postdata(); ?>
</ul>