Each storage container counts as one piece of furniture. There are three types of storage containers: Cupboard, Chest, and Shelves. Your storage maximum is for both all of your Crafted Items and the maximum amount you can of each Rare Resource. Rare resources do not occupy storage space, but they are capped at your maximum storage space. Placing a storage container into your warehouse does not destroy equipment or rare resources. For example, let's say the only containers you have are two 21-capacity chests, filled with 30/42 pieces of equipment, and that you have 25/42 of each rare resource. Placing a chest into your warehouse will make 30/21 equipment and 25/21 of each rare resource.
Storage containers all upgrade the same: