Hi, this will depend on the natural anatomy of the patient.
A weak/receding chin may need more fillers compared to someone who already has a good chin profile and just needs slight enhancement.
As mentioned by Dr Wong, an experienced doctor with a good aesthetic eye will be able to tell roughly how much is needed. It is also important to discuss with your doctor your own expectations of how you would like your chin to look.
Overdone chins tend to look either very sharp, lengthy or overtly heavy. If HA fillers were used, this can easily be dissolved to look more natural.
Hope this helps!