So demonic activity still exists today but does it take the same form as in biblical times? Why does it seem like there were a lot more demonic possession in Jesus’s day then now? Or am I being ignorant and missing demonic activity around me and just recognizing it (or attributing it to my flesh or satan himself or just consequences of sin).
Yes. Demonic activity still exists today. Though we may not see it (or be aware of it) here in the US, there are many places in the world where the demonic is much more commonplace. Reasons, why there's less appearance of demonic activity in the US, are all somewhat subjective. As you say, it can even be hard for us to distinguish if something is the work of the demonic or simple sin on our part.
When the pigs died. Did the demons die. If not where did they go?
There's no indication that the demons died (I mentioned that their eternal judgment comes later). What happens to the demons after the pigs jump into the sea isn't something we're told in the story.
Why does Matthew 8:28-34 say there were two men with demons but Mark and Luke say there was one man? Is there any information about the other man?
There are a couple of ways to reconcile this. First, some (a minority) think these are describing two different events. Notice Matthew mentions a different town (similar spelling, but not the same). So it could be two different, but very similar, events. More likely, though, Matthew is describing the same event. These thoughts aren't original to me but know that these differences are not contradictions. For instance, notice that Matthew's story doesn't mention anything about the men after being healed? Whereas Mark and Luke do. What does this tell us? It seems as if Mark/Luke have a different point in telling the story. They are wanting us to see the change in a man who is healed by Jesus. Matthew seems to want to just tell us that Jesus healed people who had demons. Why does this matter? If Mark/Luke are wanting to show the change in one man who is healed by Jesus, even if another is present and is healed, they may not include him in their story because their point is to show the change in the man who believes in Jesus (not to highlight that two men are healed; one who believes and one who doesn't).
But is there a contradiction here between Matthew and Mark/Luke? No. An example may help. Two apples are 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. But what about these two? 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. We find something similar in the gospel accounts of the story.
Finally, in my sermon, I talked quite a bit about the loneliness of the man. Even if another demonized person lived out among the tombs with him, I don't know that either of them is any less lonely. No one can help them. No one has been able to cure them. Their torment, pain, and relentless anguish are agonizing. Living in close proximity probably didn't help their loneliness.
Are there any demons described in the Old Testament? If so, where?
An example that comes to mind is in 1 Samuel 16:14-16, 23 where Saul is tormented by an "evil spirit" (that's sent from the LORD!). For more study, this is a helpful paper on demons in the Old Testament.