What is the maximum number of hair grafts for eyebrows in one session?

There is no maximum number in an eyebrow transplant surgery.  It has to do with your goal.  A typical eyebrow transplant surgery may require 300+ graft for each eyebrow. 

You should post some eyebrow photos of yourself if you
are interested in having that done.  It
has to do with the size of the area that needs to be filled and the existing
hair in the area.  On average we
transplant somewhere between 250 to 300 grafts per brow.  Maximum is up to 400 each side.

This would depend on the desired density for your brows. To recreate a brow from a minimal density, typically 300-400 grafts are required per brow depending on facial dimension and proportions.