The actual part number will vary depending on the environment. A-Flex-3 is the most common top level, from which you would select on prem device registration A-Flex-P-DEV. However if the environment is an EA, Educational, Gov, ect... it could change the part number you would use. I would recommend reaching out to a Cisco Partner or Distributer to help purchase the correct one for your needs. If you are currently on perpetual licensing then you would need to create a Subscription for this new license. You don't need to shift over the rest of the licensing but the new license purchases are under the Subscription model.
