Dynamically add form fields based on user input

Great question and here is the best way to think about it. You want to collect a set of data, but don't know how many rows the user may have. 

Use the repeating section control and put your controls within this. This will allow the user to determine, one or 100 rows as needed (i.e. dynamically), but done adding one row at a time. 

Here is a post on setting up a form with a repeating section

https://community.nintex.com/t5/Template-Gallery/Build-a-Nintex-form-pt-2-of-3/ta-p/86100

Here is another example https://community.nintex.com/t5/Community-Blogs/How-to-get-data-from-a-form-repeating-section-in-O365/ba-p/89490

Here is a post on getting data out of a repeating section

https://community.nintex.com/t5/Nintex-for-SharePoint/Repeating-Section-in-Nintex-Workflow/m-p/69867