Never write triggers that way. A trigger must perform correctly whether 0, 1 or many rows are updated. In this case, if more than one row is updated the INSERTED table will contain multiple rows but only one (arbitrary) value will get assigned to @Ref_No.
Also, ask yourself why you can't do whatever task this trigger performs in a normal stored proc rather than a trigger. In general, avoid using triggers for any process that updates or modifies data. It's usually much better to do it in a normal stored proc rather than a trigger.