If you started with 20 and the scarf is supposed to stay straight, then you should still have 20. Somewhere along the way you're adding stitches. There is a 'sticky' about unwanted extra stitches.
Often, if you're knitting every row, the error comes in at the first stitch. It gets kind of wrapped around the needle so appears to be 2 stitches instead of 1. If you knit 2 into it, you'll increase.
Make sure the yarn is hanging straight down and look for he single stitch.
When you can post a picture, we can be of more help if this doesn't help the problem.