Write a question about the following article: The Burmese too had begun to assemble their invasion force, starting during their new year celebrations in April 1759, gathering troops from all over Upper Burma, including from recently conquered northern Shan States and Manipur. By late 1759, Alaungpaya had massed a force of 40 regiments  at Yangon. Of the 3000 cavalry, 2000 were Manipuri "Cassay Horse", who had just been press-ganged into Alaungpaya's service after the Burmese conquest of Manipur in 1758. The Burmese battle plan was to go around the heavily defended Siamese positions along the Three Pagodas Pass-Ayutthaya corridor. They selected a longer but less defended route: Go south to Tenasserim, cross the Tenasserim Hills over to Gulf of Siam, and turn north to Ayutthaya. To that end, the Burmese had assembled a fleet of 300 ships to transport a portion of their troops directly to the Tenasserim coast. Alaungpaya was to lead the invasion personally, and his second son Hsinbyushin was his second-in-command. His first son Naungdawgyi was left to administer the country.  Also in his service were his top generals including the likes of Minkhaung Nawrahta who like all Burmese leadership had plenty of military experience. Some in the court urged him to stay behind and allow Hsinbyushin to lead the operation but the king refused.
Approximately how many of the 3000 troops in the cavalry were not Manipuri "Cassay Horse?"