There is no limit within each order and there is no limit per customer. You can keep buying 6 and getting 1 for as many CDs and as often as you like.
Example: You can buy 18 CDs in one order and take 3 free ones. And you can come back immediately (or the next day, or the next week…) and buy 18 more CDs and take 3 more free ones.
The ‘Buy 6 Get 1 FREE’ offer is limitless!