You can create an auto number field by following the steps given below:

1. Log in to CRM with Administrator privileges.
2. Click Setup > Customization > Fields.
3. Select the Module from the drop-down list.
4. In the [Module] Fields page, click New Custom Field.
5. In the New Custom Field page, do the following:
• In the Select Field Type section, select Auto number.
• In the Provide Field Details section, enter the field name, prefix, starting number and suffix.
6. Click Save.