Auto-Number field configuration
Key Takeaway
You can't edit a custom Auto Number field's Starting Number after you create it. To restart numbering, convert the field to Text and then back to Auto Number.
An Auto Number field fills in a unique number for each record automatically, using a format you define. App builders use it when records need a system-assigned identifier. The main decisions are the Display Format, the Starting Number, and what happens to existing data when you add the field or convert a field to or from Auto Number.
Where Auto Number fields exist
Auto Number fields exist on standard and custom objects, but the Auto Number data type is not available for external objects. Case, solution and contract records use standard Auto Number fields as their unique identifiers. Case Number is one example.