You can set the number of items you have for sale (or choose "unlimited") directly in the Chirpify Dashboard. If your quantity is limited, your listing will be deleted from Twitter/Facebook as soon as that limit is reached. If a user tries to buy and your quantities have been exhausted, the user will receive an "Invalid Listing" message.
On Instagram, this works a little bit differently as we will append the photo caption and notify users that the listing no longer valid. You can set your quantity on Instagram listings by including q(number) in your Instagram post. It would look like this "Whatever your caption is. $5 #instasale q5" to set the quantity to 5.