the default stock is how much the shop will keep if nobody buys or sells to it. if the quantity of an item is more than the default stock, the item will slowly disappear down to the default stock. if the quantity of the item is less than the default stock, the item will increase till it reaches the default stock. im not sure whether there is a limit, but most probably it'll be 2,147,483,647. most i've seen is 500k+, when pple sold willow longbow(u) to the charter ships.