First of all you can use the following calculation to get the status as returned
COUNT(if([Status]='Returned') then [Order ID]
Apart from that once you joined the return table to orders you are already getting order id field under returns, where you can directly calculate the number of orders returned.
It worked. Basically I have been working on QlikView in the past hence having a hard time learning Tableau syntax.
Thanks Venkat and Vladislav