If you want to use this stitch for a simple scarf, I would recommend to you to work with an odd number of stitches - it can be about 21 stitches for a scarf that is not too wide and which will still cover up your neck in the cold winter time. However if you prefer a wider scarf, then you can cast on 25, 29 or even 35 stitches. Also the thickness of your yarn will play a very important role. Just try couple of rows, you can always take it apart and start all over again, until your width is perfect. See this post with a free scarf pattern, where I used a moss stitch.
Other videos you may be interested to see:
- Stockinette Stitch
- Moss Stitch with an even number of stitches
- How To Knit A Very Easy Scarf (combination of Garter Stitches and Moss Stitches)