I think the point of the number of weapons was if the barrels or gas tube fittings melted down, he could switch weapons. If a rifle jammed, he could switch weapons. Perhaps even if the first few got hot, he could switch weapons and give them time to cool before they melted. If the audience just stayed there and didn’t figure out they should run, he could kill a lot of people and be famous.
It’s rather pathetic that he had such low expectations from his life, but I think he saw all those spares as a chance to rack up a large number of kills, almost like a video game score. Pistols might even have been for a getaway, if he ran out of ammo or targets and no one was outside his door. I wonder if he had a second room in the hotel, under a different name.
He rented the adjoining room a used that location to shoot at the fuel tanks. It would not have been a good location to use to shoot at the concert goers.