Depending on whether your phone is currently working or even in your possession, there are several ways to locate this information: Apple Serial Lookup & IMEI Checker - Reincubate
If you have previously synced the iPhone to a computer, the number is stored there. search iphone serial number
An iPhone serial number is a unique 11-character code assigned to each iPhone device by Apple. It can be found on the device's packaging, in the Settings app, or on the Apple website. The serial number serves as a distinctive identifier for the device, allowing Apple to track its production, warranty, and repair history. Depending on whether your phone is currently working
If the device is lost or damaged and won't turn on, check the box it came in. in the Settings app