[JeffF.]

The discussion at the Last Debate in RotK provides information from which a good estimate can be made for Gondor's forces in this battle. The Rohirrim casualties were about 2000 of the 6000 they brought to that battle giving a casualty ratio of 1/3. The Rohirrim probably suffered a slightly higher ratio of casualties since they were the longest engaged (from the transition from siege to open battle) but the forces in Minas Tirith and Aragorn's force from Pelagir would have suffered almost as much.

Of the seven thousand troops Aragorn leads toward the Black Gate 1000 are Rohirrim (500 on foot - the remaining surviving 3000 Rohirrim sent against the enemy force on the Druadan forest road). Aragorn selected 2000 of those he brought from the South and Imrahil was to find 3500 but also 500 more that included the Knights of Dol Amroth (a total of 4000 from the initial defenders of the city). If we apply the 1/3 ratio of casualties to both these forces the city's initial garrison should be greater than 6000 and Aragorn's force from the South greater than 3000 (allowing each force to suffer the same approximate 1/3 casualty ratio as the Rohirrim and still provide the numbers specified). The words of the narrative leads one to believe that these would have been only a part of each force so that the estimate of Minas Tirith's initial garrison's (that includes the 3000 that came in from the outlands) should be around 7000 and Aragorn's force about 4000. Other factors might have played into the size of Aragorn's and Imrahil's contingents like exhaustion. Some of Aragorn's force fought at Linhir and Pelargir before making the long hard pull up Anduin to the battle but some of Minas Tirith's force would have been equally strained having fought at Osgiliath and the Causeway prior to the siege but both these groups (say 1000 each) can be accounted for in the estimate and likely would have been allowed to remain in the city as Aragorn led relatively fresher troops to attack Mordor.

The description of the forces that followed Aragorn (but too late for the battle) support these since they would allow the city to have at least as many defenders as it had at the beginning of the siege.