The number in the righthand corner of each item card shows how many of that item you have in your inventory. When it reaches 0, you need to refill your inventory using Cash or Gold, depending on the item. By tapping the "Buy" button, you'll receive five of these items.
There's no limit to your inventory so if you obtain the same item from elsewhere (e.g. from the Season Pass) it will be added to the five you already have.
You can use the item as long as your inventory doesn't reach 0.