iPhone 13. Can you have one number on 2 phones and how do you tell

One way that comes close to this is to get a VOIP phone number, such as Google Voice (https://voice.google.com). It is free, and you can have your one Google Voice (GV) number forward to 2 or more other numbers. The call connects when you answer on either phone. You can also make outgoing calls using GV, so your contact will see the GV number in their caller ID.

No, a number can be associated with only one cell phone.

In addition to Lawrence Finch’s response, be certain each person has her/his own, unique Apple ID.

Make sure that each family member has a unique Apple ID - Apple Support