I knit hats for cancer wards a lot and often use 80-88 sts. Some I make smaller. Any number divisible by 4 if you have K2P2 at the bottom. Mine are all done in the round so when I’m finished, I’m done. No seam.
Also, if you know for whom the hat’s being made, measure that part of the head.
I made some for good friends recently and he wanted mid ear level while she wanted below the ear. Good things to know.