This Bible contradiction is from the Skeptic's Annotated Bible.
Only one man was possessed with devils.
Mark 5:1-2 View context
And they came over unto the other side of the sea, into the country of the Gadarenes.
And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit,
Luke 8:26-27 View context
And they arrived at the country of the Gadarenes, which is over against Galilee.
And when he went forth to land, there met him out of the city a certain man, which had devils long time, and ware no clothes, neither abode in [any] house, but in the tombs.
Two men were possessed with devils.
Matthew 8:28 View context
And when he was come to the other side into the country of the Gergesenes, there met him two possessed with devils, coming out of the tombs, exceeding fierce, so that no man might pass by that way.