I am assuming that your current relationship looks like this:
WorkOrders::__pkWorkOrderID = Discrepanices::_fkWorkOrderID
If you have FileMaker 12 or newer and want to try using SQL, ExecuteSQL can be used with a count function and a WHERE clause that specifies a specific status.
You can add a new table occurrence of Discrepancies and link it to Work Orders like this:
WorkOrders::__pkWorkOrderID = Discrepanices|Closed::_fkWorkOrderID AND
WorkORders::constClosed = Discrepancies|Closed::Status
Where Discrepancies|Closed is the name of the new table occurrence for Discrepancies and constClosed is a calculation field define to return a constant value of "Closed". Since this relationship only matches to Closed discrepancies with the same work order ID, Count ( Discrepancies|Closed::_fkWordOrderID ) will return the count of all closed Discrepancies records for that Work Order.
Perfect. Used option 2. I'll have to try and learn how to do the Execute SQL someday.