All items in a container use only one common inventory slot. Besides that, the individual members of one container do not show in inventory any more, they are represented by the container. There are various container types with differing maximum capacities. Only one container of a type is effective at a time though. Containers can be purchased in potion shops. You dont have to put single items into them. Instead, you just click hold on the container item to use it. When a container is held, the number in -brackets displays its capacity (the number of items that can be held reducing the slots) and the number in ()-brackets displays the total number of that item type in inventory (all items of that type get hidden, even those that exceed the containers capacity).