Hi AGLA16,
welcome to the Apple (user-to-user) community.
Re: "Hi I would like to know if my phone case has warranty for a replacement".
How to: Find information about your warranty or AppleCare plan - Apple Support
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Re: "how to i find my serial number for my iphone 14 pro max clear case magsafe"
Seems the serial number could be recorded on the receipt, (and may be on the packaging, if you kept it)
Although:
A discussion thread on a previous model: magsafe case - Apple Community
says:
"The cases do not have serial numbers. You just provide the receipt to use the warranty."
Re: "It is an iphone 14 pro max clear case with magsafe".
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If you want to chat with a salesperson: there's a "need some help: contact us" link if you scroll down a bit,
on this webpage: iPhone 14 Pro Max Clear Case with MagSafe - Apple
or
If you decide to visit an Apple Store: a Genius Bar reservation is recommended:
Genius Bar Reservation and Apple Support Options - Apple
or
One stop for technical support, Apple hardware service, and software support.
Call Apple Support at 1-800-APLCARE (or use the app)
AppleCare Products - Apple
All the best :-)