In The Lord of the Rings: The Return of the King, six thousand riders accompany Théoden to Gondor.
The force is the host of Rohan, summoned after Gondor’s beacons signal that Minas Tirith is in danger. Théoden gathers the Riders of Rohan at Dunharrow and leads them toward the Pelennor Fields. The number is stated in the film as the size of the force that will ride to Gondor’s aid.
The arrival of the Rohirrim changes the course of the Battle of the Pelennor Fields. Their sudden charge attacks the forces besieging Minas Tirith and demonstrates the military importance of Rohan’s cavalry. The six thousand riders should not be confused with the entire population or army of Rohan, nor with the later reinforcements led by Éomer. The battle remains costly: Théoden is killed, and the Witch-king later confronts Éowyn before Aragorn’s reinforcements arrive.