I have seen people lose up to 30 lbs and become unhappy with their facial volume after having had fillers. It is likely to have an effect, but maybe not enough for you to be unhappy. With the third injection sesion there is usualy more of a response than with the second, so you could always have more.
There is no way to know how your face will react to losing the weight. Some people lose a fair amount of volume in their faces when they lose weight and others do not. If you lose volume in your face you can have more Sculptra injected as you desire.
Sculptra is a wonderful product and adds volume that can last up to two years. However, if you are going to lose a significant amount of weight, there is no way to predict how much your face will change. Many people lose a significant amount of fat from their faces when they lose weight. You may indeed need more filler after your weight loss. Please find a board certified physician in your area with a great deal of experience in cosmetic fillers for the best result.
I would say that with a 20 lb. weight loss, that it is possible this could affect the appearance of your face. If so, it is possible that you may want more volume placed into your face after the weight loss. There is no way to know for sure, so I would just re-evaluate your appearance after you lose the weight. You could then have more Sculptra done if needed.
If you plan to undergo a significant weight change (20 pounds is significant), it would be prudent to hold off on any irreversible treatments until afterwards since facial fat volume will vary based on your total body weight.