Mark tells us that Jesus performed an exorcism on a man at Gerasenes. This is Mark 5:1-2:
Luke 8:26-27 agrees with Mark that there was only one demoniac:
Matthew goes his own way and makes it two demoniacs. This is Matthew 8:28:
It's open to you to say that Mark and Luke do not explicitly rule out the presence of the second one that Matthew mentioned. However, this would not be the only time this happens, and the more times you have to say it, the less plausible it becomes. Note that it always seems to be Matthew that's adding stuff. It's a habit of his.
Looking Unto Jesus claims that Matthew's gospel is more accurate, since Matthew was an apostle of Jesus and was actually present at the incident; however the mainstream view of academic Biblical scholarship is that the gospel of Matthew was not written by the apostle Matthew. Indeed, it's very well established that the author of Matthew used Mark as a source, which he wouldn't have had to do if he had actually been an eye-witness to the life of Jesus. If you want a convincing argument for the primacy of Mark, see Fatigue in the Synoptics, a serious work that far surpasses us amateurs.
By the way, you may have noticed that Matthew places the incident in Gadarenes. There is in fact a considerable amount of variation in the ancient manuscripts for all three gospels as to whether this took place in Gadarenes, Gerasenes, or Gergesenes. The ESV's notes reflect this (see links above). However, the context makes it clear that this is the same incident: in all three gospels, it is immediately after Jesus calmed a storm on the lake.