How many grafts do I need to fill in the eyebrow? (photos)

Thanks for the question. Based on your photos, the 100-400 grafts might be good option for you, but it would be wrong to say something for sure. You should see a board certified surgeon. You can then plan the process together and discuss the results. I wish you all the best. 

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. The number of grafts needed would be affected by the sparseness of the pre-existing eyebrow, and the desired goal for shape and size. Additionally, the thickness of the hair used will influence this. Using body hair for instance with thinner hair caliber may call for the use of more grafts. 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 provided a link with more information on the eyebrow transplant procedure, as well as a video example. 

You will have a wide variety of opinion for each doctor your consult.  It isn't only the NUMBER you should focus on, it is how it will look in the end.  It isn't just about filling in the missing spots, but filling in the adjacent areas to make it blend in with your normal eyebrow hair.Don't worry about the number, if you trust the doctor, let the doctor do his work.  You're not paying per graft for eyebrow surgery so it should not matter.No matter how great the surgeon, how perfect the surgery, transplanted hairs may look different and stand out against your normal eyebrow.  If done right, it will be an overall improvement.  If done wrong, it will stick out.

Even though it seems 30-35 would do nicely, you can't go wrong with 20. The only downside if you don't like the density (not thick enough) is you'd need to add more in the future. Anyone with a scar needs to keep in mind that grafts survival or graft uptake might be less than if there was no scar tissue. In other words, instead of 92-94 % of hairs surviving, it can range 60-80 %. Of the 20 hairs that are put in, it's possible only 12 survive.  If you put in 35 and 60-70 % survive, you'd reach 20, which is your personal target number. So keep this in mind when planning the procedure. Anyone with scars needs to be aware of the possibility of reduced graft survival and the possible need to do a second touch up session down the road. 

When doing small incisions for eyebrow you will be surprised that twenty one hair units don't cover much.  It is better to do more than do less and make it look thin again.  Forty to fifty one hair units using small incisions sounds about the correct amount that you need. 

If you look at a normal eyebrow, the hair count in the eyebrow is about 200-250 hairs. So if you needed an entire eyebrow, then this is the number of grafts that would make is look very full, assuming you don't have any eyebrow hair. So calculate the length of the eyebrow and the calculation will tell you how many hair are the right number. For 1/4th of an eyebrow, 50 hair grafts is the correct number. From the photos I would assume that 20-30 might be the right number. It would depend upon how coarse or fine you hair is. The coarser the hair, the less hair is needed.