Always start with one syringe and see if you are satisifed with the desired result. You can always add more.
There is no pat answer to the question of how much fuller is needed for a particular area like the corners of the mouth. It depends on how severe your problem is and the cause. Down turned commisures (the mouth corner) are usually multifactorial: volume loss in the cheeks and skin above the upper lip above, in the lip itself, and in the pre-jowel sulcus/chin below and excessive muscle pull down by the depressor muscles of the chin. A combination of fillers and toxins may be necessary. If the downturn has created almost scar like skin in the crease, multiple treatments may be needed over time to soften it and avoid creation of a 'donut' effect around the crease with filler.These are all issues that should be discussed with your physician at consultation. Wishing you the best of luck.