/**
* WooCommerce Extra Feature
* --------------------------
* put in functions.php file of main theme
* Change number of related products on product page
* Set your own value for 'posts_per_page'
*
*/ 
function woo_related_products_limit() {
  global $product;
  
  $args['posts_per_page'] = 6;
  return $args;
}
add_filter( 'woocommerce_output_related_products_args', 'jk_related_products_args' );
  function jk_related_products_args( $args ) {
  $args['posts_per_page'] = 4; // 4 related products
  $args['columns'] = 2; // arranged in 2 columns
  return $args;
}