Your character starts with the ability to hold 30 different items. Each item shelf allows you to hold 10 more. With 5 of this shelf your max unique items held will be at 80. If combined with the max number of item shelves you can hold a total of 110 items.
The different items total is: Special Items + Support Items + Town Items + Raw Items. This does not mean how many of each item you are holding (which is a max of 9 for each different item - unless you have the Storage Chest).
This can be bought at the Antiques Dealer for 1400 AP.