When the media uses the word "arsenal" it's one of their buzzwords to illicit a negative reaction for public. Just like "assault rifle", or "high-powered rifle", or any other terms or words they use to portray guns owners, any gun owner, in negative light.
For an arsenal as a quantity I would assume a rack(s) with a couple hundred of the same type of rifle. I would also assume dozens of other "weapons" in crates for various other spots on teams. For the amount of ammo, it would have to be measured in pallets.
No Rambo shit here. In my opinion the transition from a personal collection to a private arsenal probably tips at about 250 items.
That is just spit balling. I have seen pictures from WW1 and WW2 that had stacks and rows with probably thousands of rifles. I think the definition of arsenal is way over used and should only apply to the cooperate or government level of storage. More like a place than a quantity.
It is like dumping out 2 of the 550 Federal 22 boxes and saying "he had over a thousand boolets"