Which field should I use which can generate auto-running number?

Which field should I use which can generate auto-running number?

You can use any of the following widgets:

https://widgets.jotform.com/widget/autoincrement

https://widgets.jotform.com/widget/random_value_generator 

To add a widget, simply follow the steps from this guide: https://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form.