Tracking A persons location by cell phone number can be done my mobile apps if you have the app installed on the persons cell phone and on your cell phone as well.
But if you don't know the person or have any apps installed on the persons cell phone to track their cell phone then it's not possible for yourself to track someone's location by cell phone number.
If you do need to find someone that has become lost then you can contact the police who can then summon the cell phone provider to get GPS details of the persons cell phone and location to potentially find the person using the persons cell phone number.