Understand auto number field
The Auto Number field enables you to associate a sequential numeric value to each entry your users submit through your form. The number held by this field is automatically assigned immediately after your users submit an entry through your form, thus the auto number is a field for which your users cannot enter an input.
Understand adding the auto number field to your form
When you add the auto number field to your form, you need to set its Start Index - the number, starting from which, the auto number field will sequentially assign a number to each entry your users submit through your form. If your form already contains records, a number will first be associated with all the existing records, with the first record submitted through that form getting associated with the Start Index value.
The auto number field can store a number that contains a maximum of 19 digits. Thus when you add an auto number field, you can set the Start Index with a number that has a maximum of 18 digits.
With respect to the user-experience of the number field on a form, you can customize the field name and field link name of this field.