Please note that it may not work for all themes because of the way they are coded.

Add this code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Dessky Snippets plugin. Please don’t add custom code directly to your parent theme’s functions.php file as this will be overwritten completely when you update the theme.

/**
 * Change number of related products output
 */ 
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', 20 );
  function jk_related_products_args( $args ) {
	$args['posts_per_page'] = 4; // 4 related products
	$args['columns'] = 2; // arranged in 2 columns
	return $args;
}