https://www.gotquestions.org/one-two-demoniacs.html
Question: "Why are there two demon-possessed men in the Gerasene tombs in Matthew, but only one in Mark and Luke?"
Answer: The three passages that describe the incident with the demoniacs in the country of the Gerasenes, also called Gadarenes, are Matthew 8:28-34, Mark 5:1-20, and Luke 8:26-39. The Matthew account mentions two demon-possessed men, while Mark and Luke only mention one. Is there a discrepancy in these accounts, and do the Gospel writers contradict one another?
The first thing to determine is whether the three writers are describing the same event. The timing of the event in all three accounts—immediately following the calming of the storm on the sea of Galilee—as well as other similarities (living in the tombs, the ferocity of the demoniac, the conversation with the demons, the driving of them into the pigs, the drowning of the herd, and the response of those who witnessed the scene) all give credence to Matthew, Mark, and Luke all describing the same event. The question remains, then, whether there was one demoniac or two.
Matthew tells us there were two demoniacs, while Mark and Luke only mention one of the two. It is unclear why they chose to mention only one, but that does not negate the possibility of a second demoniac being present. Mark and Luke do not say there was “only one” demon-possessed man. They simply state that one of the two met Jesus and spoke to Him. For whatever reason, Matthew simply gives us more information than Mark and Luke.
In any case, no contradiction exists. A contradiction occurs only if one statement makes the other impossible and there is absolutely no way for them to be reconciled. For example, let's say we put two apples on a table. Statement 1: There are two apples on the table. Statement 2: There is only one apple on the table. These two statements contradict each other. Now read these two statements: Statement 1: There are two apples on the table. Statement 2: There is an apple on the table. These two statements do not contradict each other. In the same way, the biblical accounts do not represent a contradiction. All three accounts describe demon possession and the power that Jesus has over the spirit world. All three tell us that He made a point to cross the sea to save someone from the demons. All three affirm that there was at least one man who was plagued by demons. The fact that the three accounts differ in some minor details only proves that they were written by three different authors, each of whom chose to focus on a different aspect of the account.
My response
My response on this is similar to that on healig blid men in Jericho.
The verses
Matthew 8:28–34
Jesus Heals Two Men with Demons
28 oAnd when he came to the other side, to the country of the Gadarenes,5 two pdemon-possessed6 men met him, coming out of the tombs, so fierce that no one could pass that way. 29 And behold, they qcried out, “What have you to do with us, rO Son of God? Have you come here to torment us sbefore the time?” 30 Now a herd of many pigs was feeding at some distance from them. 31 And the demons begged him, saying, “If you cast us out, send us away into the herd of pigs.” 32 And he said to them, “Go.” So they came out and went into the pigs, and behold, the whole herd rushed down the steep bank into the sea and drowned in the waters. 33 The herdsmen fled, and going into the city they told everything, especially what had happened to the tdemon-possessed men. 34 And behold, all the city came out to meet Jesus, and when they saw him, uthey begged him to leave their region.
Mark 5:1–20
Jesus Heals a Man with a Demon
5 uThey came to the other side of the sea, to the country of the Gerasenes.1 2 And when Jesus2 had stepped out of the boat, immediately there met him out of the tombs a man with an unclean spirit. 3 vHe lived among the tombs. And no one could bind him anymore, not even with a chain, 4 for he had often been bound with shackles and chains, but he wrenched the chains apart, and he broke the shackles in pieces. No one had the strength to subdue him. 5 Night and day among the tombs and on the mountains he was always crying out and cutting himself with stones. 6 And when he saw Jesus from afar, he ran and wfell down before him. 7 And xcrying out with a loud voice, he said, “What have you to do with me, Jesus, ySon of zthe Most High God? aI adjure you by God, do not torment me.” 8 For he was saying to him, “Come out of the man, you unclean spirit!” 9 And Jesus asked him, “What is your name?” He replied, “My name is bLegion, for we are many.” 10 And he begged him earnestly not to send them out of the country. 11 Now a great herd of pigs was feeding there on the hillside, 12 and they begged him, saying, “Send us to the pigs; let us enter them.” 13 So he gave them permission. And the unclean spirits came out and entered the pigs; and the herd, numbering about two thousand, rushed down the steep bank into the sea and drowned in the sea.
14 The herdsmen fled and told it in the city and in the country. And people came to see what it was that had happened. 15 And they came to Jesus and saw the demon-possessed3 man, the one who had had cthe legion, sitting there, dclothed and in his right mind, and they were afraid. 16 And those who had seen it described to them what had happened to the demon-possessed man and to the pigs. 17 And ethey began to beg Jesus4 to depart from their region. 18 As he was getting into the boat, the man who had been possessed with demons begged him that he might be with him. 19 And he did not permit him but said to him, “Go home to your friends and ftell them how much the Lord has done for you, and how he has had mercy on you.” 20 And he went away and began to proclaim in gthe Decapolis how much Jesus had done for him, and everyone marveled.
Luke 8:26–39
Jesus Heals a Man with a Demon
26 bThen they sailed to the country of the Gerasenes,3 which is opposite Galilee. 27 When Jesus4 had stepped out on land, there met him a man from the city who had demons. For a long time he had worn no clothes, and he had not lived in a house cbut among the tombs. 28 When he saw Jesus, he dcried out and fell down before him and said dwith a loud voice, “What have you to do with me, Jesus, eSon of fthe Most High God? I beg you, do not torment me.” 29 For he had commanded the unclean spirit to come out of the man. (For many a time it had seized him. He was kept under guard and bound with chains and shackles, but he would break the bonds and be driven by the demon ginto the desert.) 30 Jesus then asked him, “What is your name?” And he said, h“Legion,” for many demons had entered him. 31 And they begged him not to command them to depart into ithe abyss. 32 Now a large herd of pigs was feeding there on the hillside, and they begged him to let them enter these. So he gave them permission. 33 Then the demons came out of the man and entered the pigs, and the herd rushed down the steep bank into jthe lake and drowned.
34 When the herdsmen saw what had happened, they fled and told it in the city and in the country. 35 Then people went out to see what had happened, and they came to Jesus and found the man from whom the demons had gone, sitting kat the feet of Jesus, lclothed and in his right mind, and they were afraid. 36 And those who had seen it told them how the demon-possessed5 man had been healed. 37 Then all the people of the surrounding country of the Gerasenes masked him to depart from them, for they were seized with great fear. So he got into the boat and returned. 38 The man from whom the demons had gone begged that he might be with him, but Jesus sent him away, saying, 39 “Return to your home, and ndeclare how much God has done for you.” And he went away, proclaiming throughout the whole city how much Jesus had done for him.