How much Voluma is required to see a difference?

I normally start with at least two syringes, one on each side of the face. It is a good place to start and if you need more you can always add to it in the future. I think less than one syringe on each side will not be very noticeable for most patients.

At age 28, most
patients still have a lot of facial volume. Starting at age 30, we
begin to lose a teaspoon of fat each year from the face, so by the
time we are in our mid to late thirties, two or three syringes may be
necessary. As a general guide, most patients will require one
syringe per decade, but it certainly does not need to be added all at
once. Faces come in all shapes and sizes and some will need more,
others less. In any case, it is difficult to give you a definitive
answer without seeing photos. However, in general I think it's best
to start out conservatively with the hyaluronic-acid based fillers,
such as Voluma. It is easier to add more than it is to go too far and
have to dissolve the filler with hyaluronidase.

From your
description and at your age, one syringe sounds like a good starting
place, as long as you realize that you may not see a dramatic
difference. If the person you consulted is unwilling to honor your
wish of starting slow, then I recommend finding a board-certified
dermatologist who is expert in contouring the face with Voluma who
also believes in a stepwise or “less is more” approach.  

Thank you for your question. It would be helpful to see photos. More importantly, you should feel comfortable with your board certified facial plastic surgeon. The surgeon will recommend a treatment amount based on your unique face and its unique needs rathen than setting a standard for all patients. 
Best wishes,