What are the accurate areas for Botox injection to treat gummy smile? And units required for?

This treatment can't be done without affecting your smile.  Folks really do not like the effect no matter how great the doctor thinks they are.  The so-called gummy smile is a big beautiful smile.  It is not an abnomality that needs to be treated.

As a woman I totally understand the importance of well contoured lips and smooth skin!
As a dermatologist, I can tell you that "around the mouth" can include different injection methods to achieve different results. Some injections can decrease wrinkles around the mouth (smoker lines), treat gummy smile, downward angles of the mouth and so on.

The key is to make sure you communicate what you are trying to achieve and ask specific questions from the doctor as the risks for that treatment.

I do many gummy smiles and they can be successfully treated with Botox/Dysport when performed by expert injectors. Injection of a low dose into precise points on the upper lip to reduce the “gum” showing without affecting any other motion. I have my patients smile several times, and animate their face as they are talking to me during the consultation. Where exactly I place it (and how much product) depends on the anatomy of the patient.
Factors to consider include how thin/thick the upper lip is (and if you use fillers on the upper lip you need to consider balancing the lower lip to avoid the "duck lip" tragedy!). How much the lips are pulled up and how many millimeters of gum are exposed. Any asymmetries during movement or at rest will also have to be taken into account.
I think the key is the consult to show you in the mirror what to expect. In this case Botox/Dysport may lower the upper lip so that you are not be able to show the gum line when you smile.  Because there is some risk of lowering too much, (which could affecting your singing or playing of a wind instrument, for example), using low doses and making sure that you receive precise injections by a highly qualified and certified professional are very important!
Best of luck!