To increase the number of related products shown we need to edit the file - shop.product_details.php

Open file joomla_root/administrator/components/com_virtuemart/html/shop.product_details.php and go to line 85.
Change the number of returned results in the query -
 
$q .= "AND FIND_IN_SET(#__{vm}_product.product_id, REPLACE(related_products, '|', ',' )) LIMIT 0, 4";
 
to for example 8
 
$q .= "AND FIND_IN_SET(#__{vm}_product.product_id, REPLACE(related_products, '|', ',' )) LIMIT 0, 8";