how to find an iPhone with serial number

The serial number will only help if someone finds it and checks to confirm that it is yours. Check this link...

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

There is no way to find an iPhone using its serial number (or IMEI, for that matter). You can only find an iPhone if Find My was enabled on it and it has an Internet connection, or if you also enabled the Find My network and another iPhone detects its presence and reports it.