I lost my iPhone I need serial number to find it how can have serial number??

you should be able to go to supportprofile.apple.com and find your serial number there, but i'm not sure how the serial number will help you find the phone.

iOS: How to find the serial number, IMEI, MEID, CDN, and ICCID number

You can also call your provider and ask them for the IMEI number ,, they may also have your serial number. and if you have the box the serial number is on the box as well