The slaves were probably a mixture of orcs and other races. There might not have been many orcs left at all; in Book VI, Chapter 4, it is said that many of the

The slaves were probably a mixture of orcs and other races. There might not have been many orcs left at all; in Book VI, Chapter 4, it is said that many of the orcs after Sauron's fall slew themselves or fled away to hide, with the implication that they would not have survived for very long without Sauron's direction.