Adding an option to automatically fill (a number)

It's not an automatically answer.
It's a document.

If you are trying to update a field value based on an entry in another field, that can be done using conditional logic:

http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic

http://www.jotform.com/help/286-How-to-Populate-Field-with-a-Value-from-another-Field

I am not sure what you are referring to by "It's a document".  If you wouldn't mind elaborating on what you need done with the document, we will be happy to assist you further.