As a member of the iPhone Developer Program you are assigned 100 device "slots," allowing you to add up to 100 devices in the iPhone Program Portal in a membership year. Adding a device registers it to be used for developing and testing apps, or for sharing your app with other users via Ad Hoc Distribution. If you reach the 100-device limit and wish to add more, you can remove devices you are no longer using when your new membership year begins. At that time, you can remove as many devices as you like and then recycle those slots for new devices.
The ability to recycle slots is only available from the start of a membership year until the time you add your first new device, so be certain to remove all of the devices you no longer require before adding any new ones. Once you've added a new device, you will be unable to recycle additional device slots until your next membership year begins. If you renew your program before your expiration date, the recycling period does not start until your membership anniversary date. If you renew your program after expiration, the recycling period starts immediately.