At this time, you can't.
In Cheqroom, all Items are unique and physical pieces of equipment.
Although it makes a lot of sense to have "anonymous" Items with a quantity e.g. for consumable inventory or generic equipment, we don't have it yet. This is something we have on the product roadmap.
As a workaround you can use the Item duplicate function to quickly create a lot of identical items. Each of the items will be uniquely identifiable and trackable.