To change the number of related products only you can use this code in the Theme Functions (functions.php) file:

/**
 * WooCommerce Extra Feature
 * --------------------------
 *
 * Change number of related products on product page
 * Set your own value for 'posts_per_page'
 *
 */ 
function woo_related_products_limit() {
 global $product;

 $args = array(
 'post_type' => 'product',
 'no_found_rows' => 1,
 'posts_per_page' => 6,
 'ignore_sticky_posts' => 1,
 'orderby' => $orderby,
 'post__in' => $related,
 'post__not_in' => array($product->id)
 );
 return $args;
}add_filter( 'woocommerce_related_products_args', 'woo_related_products_limit' );

Note: This change may be overwritten by a Theme update.