How many units of Botox are required to eradicate the sagging skin on the jowls?

The answer is NO.  Jowls develop as cheek skin and fat sag forward and collect just behind the mouth.    This leads to deepening of the nasolabial and marionette folds and bulging of jowl fat outward.  The underlying cause is looseness and therefore some type of tightening is needed to improve this condition.  Both surgical (cheek lift) and non-surgical (various technologies such as light based, ultrasound, and radiofrequency based) treatments offer different degrees of tightening.  Honestly, surgery tightens and smooths the tissue the most, and the non-surgical treatments offer just modest tightening.  Also, injection of fillers in the depressed folds in front of the jowl bulge can help smooth the transition into the jowl and camouflage the area.

Botox wouldn't work for the jowls. Consider fillers instead, which would replace volume loss and plump up the face to smooth away jowls.

I am unaware of botox being use to treat sagging skin. I would check the source very carefully if someone is offering that treatment to you.