Among the top 5 heaviest shipments, how many shipments were transported via Mack?
heaviest shipment refers to Max(weight); via Mack refers to make = 'Mack'
SELECT COUNT(T2.ship_id) FROM truck AS T1 INNER JOIN shipment AS T2 ON T1.truck_id = T2.truck_id WHERE T1.make = 'Mack' ORDER BY T2.weight DESC LIMIT 1