Not a linguist here, but I think the term originally referred to a facility and not a specific collection. In other words the building armaments are stored or serviced in and not the actual items.
However, it has come to mean any collection of firearms and I don’t think there is a specific size. Obviously it is used to sensationalize a story and another term would probably be more accurate, but not sure it is really wrong as far as modern usage goes.
A major source of objection to a free economy is precisely that it ... gives people what they want instead of what a particular group thinks they ought to want. Underlying most arguments against the free market is a lack of belief in freedom itself.
- Milton Friedman