How can I create a fillable form with 2 side by side columns and 20 rows? Thank you!

You may try using the input table in your web form.

You can add columns only and blank spaces for your rows.

Sample output:

You can then download it as a PDF form and convert into fillable PDF.

However, I would suggest using the PDF document instead ,if you don't have a fillable PDF as an outline.

Guide:https://www.jotform.com/help/384-how-to-customize-pdf-submissions-report/

Let us know how it goes.