One stamp for the correct amount of postage is all that is needed to mail a CD. Stamps for any value can be purchased online or in person from any carrier company, including the United States Postal Service, UPS and Federal Express.
CDs fall into the realm of media that can be mailed under the Media Mail price through the USPS. This is oftentimes a more cost-effective method of mailing packages than using standard postal rates when sending a package weighing more than 2 pounds, but the package must be marked "Media Mail." Since CDs weigh less than a pound each, mailing a single CD with regular postal rates may cost less than the Media Mail price.
Stamps come in a wide variety of values, so any combination of them totaling the necessary price of the postage would be acceptable. Prices for postage can vary between carriers, and pricing tools are available online at each carrier's website for the comparison shopper. All that is needed to determine the rate of postage is to know the weight of the package and the departure and destination mailing addresses. Once the rate of postage is determined, the number of stamps on the package are irrelevant, so long as they total the necessary cost of delivery.