How do I add a hidden field that auto populates in numerical order each time a form is submitted?

https://www.jotform.com//?formID=63194536956973

I believe you are looking for the following widget that adds unique id for each submission.

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

Please refer to the guide below that can help you with adding unique id on your form.

https://www.jotform.com/help/79-How-to-Add-Custom-Unique-IDs-to-your-Form-Submissions

Hope this information helps!