Looks like seed
stitch. The best way to figure out how many sts to cast on is part of the answer to you next question.
6 stitches and 12 rows = 4 inches in Seed Stitch"
You should do a gauge swatch using your yarn and needles. Cast on about 10-12sts and work in seed st pattern for about 5-6 inches. Then measure how many sts you get over 4 inches. Usually you would change needle size to get to the pattern gauge of 6sts per 4 inches but for a scarf like this, you can just cast on the number of sts you'll need for the size you want based on the number of sts you get per 4 inches.
Take a scarf you have and wrap it around your neck the way you would like the infinity scarf to wrap and then measure how many inches long (or wide) the scarf is. Use that for your infinity scarf size. (Your pattern probably also gives you these measurements.)