Do you mean the IMEI number?
If so if you still have the box or the plastic rapper from you phone then check them. The IMEI number is also printed on them.
You also can login on
windowsphoe.com and log in with the account you used on your phones and ther you will see your phone and the IMEI number.
You bye the way can not track your phone with the IMEI number you only can use it so the police our your provider can block the phone so it can not be used anymore.
To track your phone you must have enabled the "Find my phone" under settings if so you can track your phone also in
The Smartphone Reinvented Around You | Windows Phone (United States) (and if location is enabled the tracking will be much better also the phone should be ON)
Good luck and I hope you find your phone.
If you need more assistance pleas register and reply on this post.