Eyebrow transplant; is less than 100 grafts possible?

It is up to the doctor.  There is no set rule.  There is no minimal requirement.  Eyebrow surgery may take less than 100 grafts, even less than 50 grafts. Each patient is different.The issue is what the doctor will charge for surgery.  There is usually a minimum fee.

The average number of grafts needed for eyebrow transplantation tends to range from 50-325 grafts. It is certainly possible to undergo this type of surgery with around 80 grafts to achieve your desired outcome and look your very best. Using a very high number of grafts may produce a harsh looking result. 
If you would like a smaller number of grafts, try to find a doctor who can accommodate your needs through a cost structure along these lines.

I have included a link and video with more information below. 

You can put many or as few hairs as needed, however, for many hair clinics, there is a minimum to cover the cost for supplies, time, staffing, equipment and followup.  You may be able to find a clinic willing to do 100 hairs but you will need to call around an inquire.