I acually wondered the same thing last night - people were posting quantities and I didn't ever see where they were listed. Fiddling around, I added an item to my cart, then on the screen with the quantity box, I changed the quantity to some high number. Then hit 'update quantities' or whatever and it will return an error saying the max allowable is X - where X is what they have in stock (it fills in the new max in the quantity dialogue for you).