ecommerce-functions.php

/**
 * 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['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;
}