The accounts are in Matthew 8, Mark 5 and Luke 8 and all mention a man possessed by “Legion” (for we are many). They entreated the Lord that he send them into a herd of swine. Wish granted and the herd of swine (not one) rushed into the water and drowned.
When someone makes a specious claim about the Bible contradicting itself, I usually ask two things: can you share with me one or two examples of this that you assert, AND read the accounts of the trial of Jesus before the Sanhedrin, and note how the different tellers of the story give supporting clues regarding the ‘slap’ Jesus received and subsequent demands. Those accounts were given by different people, who did not have the other aqccounts to use as reference, and were written down to help tell the story of God with us and the purpose of Christ coming in the flesh. So, contrary to contradicting each other, these separate tellings of the same event help to clarify the particulars yet the tellers had no collaboration with eacdh other in time or location when telling their version. THAT is evidence of supernatural authorship at work.