On 5 March the British expedition sailed from Spithead, England, with 7 ships of the line and 4,365 men aboard 64 transports, and arrived in Barbados on 20 April.  Five days later the expedition reached Fort Royal on the recently conquered island of Martinique where it picked up the remainder of Major-General Robert Monckton's expedition, still numbering 8,461 men. Rear Admiral George Rodney's squadron, amounting to 8 ships of the line also joined the expedition bringing the total number of ships of the line to 15. On 23 May the expedition, now off the northwest corner of Saint-Domingue , was further reinforced by Sir James Douglas' squadron from Port Royal, Jamaica. The force under Albemarle now amounted to 21 ships of the line, 24 lesser warships, and 168 other vessels, carrying some 14,000 seamen and marines plus another 3,000 hired sailors and 12,826 regulars.
Answer this question: How many ships of the line did James Douglas' squadron bring?
6