This depends on the yarn you are using and the circumference of your hat. Make a gauge swatch (in the round if possible) and calculate your gauge. That is what I did with this hat. I saw that I was getting 5 stitches for each inch (20 sts over 4") and I wanted my hat to be 12" in circumference. 12 times 5 is 60, which is how I figured out how many stitches to cast on for my newborn hat.