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.