According to TolkienGateway.net, Mordor's troops consisted of some 18,000 Easterlings and Haradrim, several Haradrim war Oliphaunts, and tens of thousands of Orcs just for the Battle of the Pelennor Fields. Estimates for the number of orcs are over 50,000, with the full army size at 75,000.
Sauron had over 60,000 orcs, trolls, beasts, and Easterlings in reserve back in Mordor. These forces were unleashed through the Black Gate at the Battle of the Morannon.
During the Battle of Pelennor Fields, Sauron also sent many Easterlings to fight in the Battle of Dale.
How did Sauron feed an army that size?
The land in the western parts of Mordor was largely infertile, producing only sparse brambles. The southern part of Mordor, Nurn, was slighly more fertile, and moist enough to carry the inland sea of Núrnen. Nurn was made somewhat fertile because the ash blown from Mount Doom left its soil nutrient rich, thus allowing dry-land farming. And according to TolkienGateway.net, the inland sea of Núrn was salty, not freshwater.
He's not growing crops and planting gardens. Nor is there enough plant life in Mordor for grazing animals like sheep or goats. Nor was the water fit for irrigating crops.
We know orcs eat (and hate) maggoty bread, but prefer fresh meat:
So what did they eat?
As usual, I prefer answers with specific quotes from the books, but I will accept quotes from the movies. My policy is that "Quotes get Votes", meaning if you provide citations and exact quotes for your answers instead of unsupported speculation, I am more likely to vote for your answer.