New As of Oct. 2014
Each perpetual license you purchase for our web-based management software lasts for the lifetime of the device it’s enrolled on.* Perpetual licenses are available for purchase by all Chrome device customers. If you encounter a hardware issue and need to replace a Chrome device, the license can be transferred to a replacement of the same model. Using thedeprovisioning feature within the Admin console on a damaged device will free up its license, allowing you to enroll the same model replacement. Licenses are not transferrable from one device model to another, and license transfers will be audited.
Google Info: Link
Notice below they do call out buffer licenses. This is typically used for repairs or dead units. Though no specifics on what the buffer amount is.
Each license you purchase for our web-based management software lasts for the lifetime of the device it is enrolled on.* Licenses are not transferrable from one device to another. If you encounter a hardware issue and need to replace a Chrome device, you will have a limited number of buffer licenses so that you can continue to use the web-based management software with the replacement device. However, you will receive an error if you exceed the buffer. If you see the error, you'll need to purchase additional licenses by contacting sales. If you are an educational organization, please contact our Education sales team. For any other issues related to enrollment, please contact support.
You can view the number of licenses currently in use in your Google Admin console. Go to Device management > Devices(to the right of Chrome management). Note that buffer licenses will not be displayed.
*Each device model is supported until it reaches End of Life. For more information about End of Life and the associated dates for each model, please visit the Chrome End of Life Policy page.