The following Python code snippet uses the JSON output to list the serial numbers of all connected devices:
import json
import subprocess
# List all devices
output = subprocess.run(["nrfutil", "device", "list", "--json", "--skip-overhead"],
capture_output=True)
stdout = output.stdout.decode("utf-8")
devices = json.loads(stdout)["devices"]
# Print the serial numbers of the devices
print([device["serialNumber"] for device in devices])