The number of stitches for your CO is going to be determined by the weight of your yarn and the size of your needles. If you use an Extra Bulky yarn or run two Bulky weight yarns together, you will likely use US 35 needles. If so, you might have a gauge of 3 sts / inch. So, for a medium width scarf, a CO of 12 stitches would be fine.
Can give you a precise answer once you have chosen your yarn(s). You will find plenty of nice patterns here and here.