I Am Getting an Fue Sideburn Enhancement, 100 Hairs 50 on Each Side; Will They Be Natural in Appearance?

FUE does not mean you'll get great results.  FUE is just a method to harvest the hair follicles. It is what the doctor does with those hair follicles that matter.It is not also about the numbers.  If you doctor believes 100 grafts will do the job then that is what it takes.FUE will leave 100 dot scars  FUSS (strip surgery) will leave a half inch (or less) line scar for 100 grafts.

Fifty follicles is not very many, but with proper angle of the incision and placement of the grafts, it may do the trick of giving the illusion of less see through sideburns.
Also you mentioned your hairline for the future so the decision on doing an FUE procedure was smart because this type of procedure you can do as little or as many hairs as you want to do. I would recommend ARTAS robotic hair transplantation for best results

You’re right, the results of this procedure are dependent on the skill and talent of the surgeon. Results are also largely dependent on the donor source (hair from the mid-rear scalp? Nape of the neck? Leg hair?). With the right practitioner and the right donor hairs, your sideburns should look totally natural. Additionally, with an experienced surgeon and only 100 extractions, scarring should be negligible. That said, even in the best of hands no result can be guaranteed in surgery.