You will need to edit the file template-portfolio-3c-full-width.php line number 19 to change the number from 6 to a bigger one.
I would recommend you load the child theme that comes with the theme bundle, create a new file with the same name in the child theme directory, copy content over to the new file from the parent theme and then edit it. This way your changes will not be lost when the theme receives updates. Thanks
The child theme is already included in the theme download bundle. You just need to upload it and activate just like any other theme. You can learn a little bit about the basics of Child theme here –