Where can I find a serial number from a previous iPhone?

Look at your receipt, log into your account to see if it is in your purchase history.

Why do you need the iPhone serial number that you do not have anymore?

They want to submit it for the court settlement.

Yes, I know. That's why I asked.

Isn't it cheating if you do not have the iPhone anymore?

What about the iPhone passed on to 5 users?

Do each can make a claim?

No, it is not cheating and you can file for any iPhone that you owned during the covered period.