Like everything else about knitting...it depends.
It depends on the size of the finished cowl, the thickness of the yarn, the size of your needles, your gauge, how loosely or tightly you knit, the stitch pattern and the phase of the moon.
Choose some yarn and use the needles recommended on the ball band. Make a good-sized swatch in the stitch you want -- about 6"x6". If the fabric looks good to you, great. If it looks too flimsy, make a swatch with smaller needles. If it looks/feels too tight, make a swatch with larger needles. Repeat until you make a swatch you really like.
Measure off 1" in the center of the swatch and count how many stitches you have in the inch. That's your gauge. (Let's say you have 6 stitches per inch.)
Now decide how wide you want the cowl to be. (Let's say you want it to be 6" wide.) Multiply the number of inches by the gauge (6x6 = 36). That's how many stitches to cast on.