How can you find out the serial number to each of the AirPod pros

Find the serial number of your AirPods - Apple Support

Read the fine print at the bottom, the 2nd Generation AirPods and AirPod Pros do have serial numbers but if you are looking for service you don't need them.

Individual Airpods do not have serial numbers. To find your serial number, you can look underneath your charging case or go into Settings > General > About > AirPods when they're connected to your device.