My understanding was that it is not the number of the item, but the number of characters who have the item at that price slot that is restricted. So one char could have thousands of one item, yet be the only char at that price slot on the market - but if another item has the max chars but each char only has one on the market, then the second item will be maxed out at that price slot with less of the item on the market.
I have great faith that more experienced, knowledgeable players will correct any misconceptions I have.;) Thanks in advance guys!!