Auto-number field configuration
Key Takeaway
You can't edit a custom auto-number field's Starting Number after you create the field. To change it, convert the field to Text and then back to Auto Number.
An auto-number field gives each record a unique number when the record is created. Salesforce generates the number automatically, in a format you define, such as Sample- 00003. Setting up an auto-number field means choosing a Display Format and a Starting Number, and knowing which choices are hard to change later.
Standard and custom auto-number fields
Some auto-number fields already exist in Salesforce. The unique identifiers for case, solution, and contract records are standard auto-number fields, such as Case Number on cases. Admins can also add custom auto-number fields or use auto-numbering for a custom object's Record Name.