Hmm Is this do-able?

Im using a slider on a theme. i am using this code.

<li class="subitem">
            <ul class="items">
              <?php if (have_posts()) : ?>
              <?php while (have_posts()) : the_post(); ?>
              <li> <a href="<?php $key="image"; echo get_post_meta($post->ID, $key, true); ?>" title="<?php the_title(); ?>">
                <?php if ( get_post_custom_values("thumb") ) : ?>
                <img src="<?php $key="thumb"; echo get_post_meta($post->ID, $key, true); ?>" alt="<? the_title(); ?>" />
                <?php else: ?>
                <img src="<? bloginfo('template_directory'); ?>/timthumb.php
?src=<?php $key="image"; echo get_post_meta($post->ID, $key, true); ?>&w=180&h=107&zc=1" alt="<? the_title(); ?>" />
                <?php endif; ?>
                </a> </li>
              <?php endwhile; ?>
              <?php endif; ?>
            </ul>
          </li>

This is the slider works based on the li class subitem. i need wordpress to create a new li class subitem every x amount of posts.