That depends on the "weight" of the yarn you're using.
But here is a good rule of thumb IMHO:
1) With dk weight yarn: cast on 25-30 st
2) With worsted weight yarn: cast on 20-25 st
3) With bulky weight yarn: cast on 12-15 st
It would be good to knit a trial "swatch" with your yarn.
Cast on 20 stitches...and knit for at least 2". Measure how many stitches equal 1" in width.
Using that information, multiply that number of stitches by how many inches wide you want your scarf to be. For example, my favorite scarf width is 5". So if my yarn is producing 5 stitches per inch...I would cast on 25 stitches to get a 5" wide scarf.
I like my scarves to be extra long, to wear doubled...therefore, 5" width is perfect.