He brought all those guns to the hotel room he shot from. I imagine it was so he could shoot as many rounds as possible at the crowd with out the need to reload.
But that really makes no sense. Unless you have them all set up in a row pointed exactly where you want, you’re probably not even saving half a second vs reloading. The old “switching is faster than reloading” thing doesn’t apply nearly as much when you’re at a static position and can have all your mags out in the open at arm’s reach.
He was operating a significant number of his weapons on bump stocks. Bump stocks allow firing at a much higher rate than the weapons were designed for. Operating at a higher rate causes the weapons to overheat. Overheating causes misfires and jams (and inaccuracy and can permanently damage weapons, but I doubt he was particularly concerned about those things). He did have them all set up in a row and many on mounts. He broke out the overlooking windows of his hotel room before he started shooting. It seems he was shooting with one until it jammed and then moving on to the next rather than trying to clear misfires.
If that is the case, that he was using a gun until it jammed, it makes more sense to me. At the same time, how often does an ordinary gun jam? I’ve used an HK416 and an MG3 during a year of army service (conscription training) and to my memory you could fire many hundred rounds (thousands in the case of the MG3) without a single jam, and a misfire takes about a second (max) to clear.
Also, I’ve seen people talking about the number of guns someone has also in other settings, as a kind of metric that people who are into guns seem to care about, I guess I’m more wondering about the phenomenon in general than just this specific case.