Use the number of stitches in the pattern repat and add more in that same increment. In the pattern you linked to, the repeat is 6 sts, so you would add 6, 12, 18, 24, etc stitches to the CO number that's given. Don't just double the cast on number because the edge stitches aren't part of the repeat. This one does happen to have 6 edge sts, so any multiple of 6 will work.
BTW, I had to go through a lot of rigamarole to sign up and look at this pattern, then it just takes you to the site it's hosted on. You can get to that site directly - the Dishcloth Boutique
- where there's 6 pages of knit dishcloth patterns.