Trying to only display 1 category using get_posts

Did you try the example given in the codex?

<?php
global $post;
$args = array( 'numberposts' => $number, 'category' => 6, 'order' => 'ASC', 'orderby' => 'post_date'  );
$myposts = get_posts( $args );
foreach( $myposts as $post ) :  setup_postdata($post); ?>
    <li><a href="https://wordpress.stackexchange.com/questions/20091/<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
<?php endforeach; ?>

This a modified version that should suit your needs. Are you also certain that this category exists?

Edit:

<?php $temp = $wp_query; $wp_query= null; $wp_query = new WP_Query('showposts=" . $numberposts . "&cat=6&orderby=date&order=ASC'); ?>
<?php while ($wp_query->have_posts()) : $wp_query->the_post(); ?>
        <!-- your content -->
<?php endwhile; ?>
<?php $wp_query = null; $wp_query = $temp; ?>