I lost my iPhone and I don’t have the box or serial number what I should do?

If you have lost your iPhone you may be able to track it, and remotely erase the device, if you had Find My enabled.

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support (UK)

You may also be able to find the serial number of your device if it was linked to your Apple ID, you may also find your serial number printed on a purchase invoice if you still have it.

Find the serial number or IMEI on your iPhone, iPad, or iPod touch