The Last Debate was the council that Aragorn held with Imrahil, Gandalf, Éomer, Elladan, and Elrohir. It was held outside Minas Tirith on March 16, 3019, and there it was decided "that they should set forth on the second morning from that day with seven thousands [of men], if these might be found" to fight Sauron in the Battle of the Morannon, in the hope that Frodo Baggins and Samwise Gamgee might finish the Quest of the Ring in Mordor.
Also debated was what size expeditionary force should be mustered, and the number to leave behind for defense. In the end they decided to leave behind 3,000 Rohirrim to attack a very large blocking force stationed on the West Road in Anórien; 4,000 who were coming by land from Pelargir; possibly several thousand who were coming up the Anduin; and probably some 1-2,000 survivors from the siege of the city. All in all the defensive forces' numbers could been upwards of 9,000, fulfilling Aragorn's plan of leaving Minas Tirith with a larger defensive force than what it had before the siege began.
The expeditionary force numbered 7,000 men. Under Aragorn's command were 2,000 of those who sailed with him up the Anduin; under Imrahil were 3,500 who were probably from among the original defenders of the city; under Éomer were 500 Rohirrim unhorsed, and another 500 horsed; and lastly another 500 horsed Knights of Dol Amroth, Dúnedain Rangers and the sons of Elrond.