The wording on their website appears to allow for multiple charger purchases for just a single "turn-in" provided you bring in the same amount of iDevices for proof of ownership.
So I can bring in a single cheap charger, and say 10 iPhones and then be able to buy 10 real chargers for my devices. Somebody go and confirm for me. :pFor a limited time, you can purchase one Apple USB power adapter at a special price — $10 USD or approximate equivalent in local currency. To qualify, you must turn in at least one USB power adapter and bring your iPhone, iPad, or iPod to an Apple Retail Store or participating Apple Authorized Service Provider for serial number validation. The special pricing on Apple USB power adapters is limited to one adapter for each iPhone, iPad, and iPod you own and is valid until October 18, 2013.