In the United States, a person's passport number changes each time he renews it. Most passport numbers have nine digits, but some have as few as six digits, according to U.S. Citizenship and Immigration Services.
Some travelers are allowed to renew their passports by mail, but others must do so in person. As of 2014, someone renewing by mail must have his most recent passport in his possession. Additionally, the most recent passport must be no more than 15 years old and have been issued to the traveler when he was at least 16 years old. Travelers who do not meet these requirements must renew their passports in person.