From https://developer.apple.com/support/ios/program-renewals.html - one to watch out for when you renew your iPhone Developer Program membership:

When Team Agents or Admins first log into the iOS Provisioning Portal at the start of a new membership year, they will be presented with the option to remove devices and restore the device count for those removed devices. Important Note: Devices can be removed and their device count restored only up until the first new device is added. Therefore, it is important to remove all devices you are no longer using for development prior to adding any new devices.