I tried following approach and it worked.
rename .twb file to .xml
(if your tableau file is twbx then rename this file to .zip . get .twb file from this zip file. )
Open in notepad
search for following text:
change ’16’ to your desired value.
Now rename the .xml back to .twb. Open in tableau.
Though you could add more columns in tableau by changing xml, UI doesn’t look good. As tableau doesn’t support scroll bar on “Rows”