To the best of my understanding, the pattern that you are working on is in multiples of 12 + 1. 12+ 12 equals 24, plus 1 equals 25. If you wanted your scarf wider you would add 12 + 24 +1; which would equal 37. I hope this helps.
Your multiple is 12 and you will have 1 stitch on each edge is what I am seeing. So if you make it 2 repeats wide, 26 is the correct number to cast on. 3repeats wide would be 36 plus 2 (1 edge stitch on each side) for a cast on of 38 stitches.
You are dealing with a pattern that requires you to multiply by 12 for every repeat you want to make, and then add 1 to your count. It has nothing to do with selvages.
If you wanted a wider scarf with 4 repeats of the pattern, you would cast on 48 plus 1 or 49 stitches.
If you look through a book of stitch patterns, you will see many patterns that are simple multiples, as well as others that specify an added number stitches, like this one (12+1) and others that say something like "a multiple of 6 +5" (lacy check pattern). Generally when a pattern adds stitches to the multiple, the result is an offset from repeat to repeat, but again, this kind of stitch count in a pattern doesn't have anything to do with borders or selvages.
If you wanted to add selvage stitches, your best bet would be to determine what kind of selvage you want, then add the stitches called for in the selvage technique and swatch the pattern.
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