How much filler is usually put in a forehead that lacks volume?

When treating forehead volume loss, I usually use 1 (1 mL) syringe of Restylane. In some cases, I will dilute the Restylane with bacteriostatic saline to make it easier to spread along the frontal bone. 

Foreheads can vary a lot!  The temples especially, can take a large amount of filler to volumize properly.   Most people will need one to two syringes, even 3,  per side.  The forehead itself is a little trickier because too much volume here can look really bad.  I wouldn't do more than one syringe at a time and build up slowly to get the best result.  Keep in mind swelling can be an issue and things will probably look worse before they get better.
Hope this helps!
Dr. Amy