Why are there so many gas stations? Were they already boarded up and empty before the war or were they still working? There isn't really any gas for them to have used either, it seems like a waste to me. I understand it could just be filler, but I want to know if anybody has an explanation for it.

There COULD have been gas. Assuming we take bits and pieces of Tactics out as canon, Gas existed, but VERY VERY expensive.

Look at Operation Anchorage, which was primarily fought for the recovery of the oil pipeline in Alaska. Operation Anchorage ended in January 2077, so there is plausibly still oil in the U.S. at the time of the War, albeit a mere trickle.

But if it's so rare why would 4+ gas stations be needed? One would do the trick.

Because Las Vegas is Las Vegas. It would have cars going in and out. If you have the money to gamble, you may likely drive if it's not to far. I honestly can't give you all the answers. But Given those roads they're on are all interstate roads, it'd be the most likely needed thing. Like IRL, you're quite likely to see two gas stations across from each other.