Add another column showing the shipping agent name
SELECT T1.shipping_agent_code, T1.shipping_agent_name, count ( * )  FROM Ref_Shipping_Agents AS T1 JOIN Documents AS T2 ON T2.shipping_agent_code  =  T1.shipping_agent_code group by T2.shipping_agent_code
add another column refers to including an additional field in the SELECT statement; showing the shipping agent name refers to selecting shipping_agent_name;