The majority of mercenaries recruited by Gustavus Adolphus were German, but Scottish soldiers were also very numerous. These were composed of some 12,000 Scots already in service before the Swedes entered the war under the command of General Sir James Spens and colonels such as Sir Alexander Leslie, Sir Patrick Ruthven, and Sir John Hepburn. These were joined by a further 8,000 men under the command of James Marquis Hamilton. The total number of Scots in Swedish service by the end of the war is estimated at some 30,000 men, no less than 15 of whom served with the rank of major-general or above. With Tilly dead, Ferdinand II returned to the aid of Wallenstein and his large army. Wallenstein marched up to the south, threatening Gustavus Adolphus's supply chain. Gustavus Adolphus knew that Wallenstein was waiting for the attack and was prepared but found no other option. Wallenstein and Gustavus Adolphus clashed in the Battle of Lützen , where the Swedes prevailed, but Gustavus Adolphus was killed. Ferdinand II's suspicion of Wallenstein resumed in 1633, when Wallenstein attempted to arbitrate the differences between the Catholic and Protestant sides. Ferdinand II may have feared that Wallenstein would switch sides, and arranged for his arrest after removing him from command. One of Wallenstein's soldiers, Captain Devereux, killed him when he attempted to contact the Swedes in the town hall of Eger  on 25 February 1634. The same year, the Protestant forces, lacking Gustav's leadership, were smashed at the First Battle of Nördlingen by the Spanish-Imperial forces commanded by Cardinal-Infante Ferdinand.
Question: How many more Scots were in Swedish service at the end of the war than there were at the beginning?
Answer: 18000