How many heads, faces, figures should you draw per day to really improve your drawing skills?

a lot of them to get it right. 

Why do you want to know how many I should draw? In the future, when asking about yourself, the correct phrasing is "How many heads, faces, figures should I draw per day to really improve MYdrawing skills?"

.

Got that? Good. 

If you make four twenty minute studies after two ten minute minute warmup/gesture sessions you will probably reach the limits of your concentration. After that there is little point in continuing until your concentration is recovered.

As many as you want, it is more important to keep doing it daily. 

As many as possible.  the more you practice, the better you can be.