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.

The key to sideburn enhancement has nothing to do with how the hair is havested on the donor site.  It has everything to do with the preparation of grafts and angle to which the hair is inserted.  If hair is put in properly it should look natural. 

We have done hundreds of such patients over the past 24 years and the sideburns look great. The key to good sideburns is the use of one hair grafts on the outer border and possibly throughout the sideburn area

Hair transplants will enhance your sideburns and look natural as long as long as the work is done by an experienced surgeon, but you will need more than 100 hairs to make a significant improvement. Be careful where you place the grafts because it is possible that you won't be able to shave that area because it is unlikely but possible the skin surface may be irregular after graft placement.

 Fifty follicles per side is not very many and I'd think you'd have to be realistic about the amount of coverage that's going to give.  

The only good reason to harvest hair trasnsplants by FUE is if you have plans to shave your head. If so, will you also be shaving the beard and sdeburns? Then, why have any transplants done at all.

Also if you are under 30 with a Norwood 2 pattern and a family history of progressive male pattern baldness, you should approach hairline  correction with the understanding that you will need future work.

The match of scalp and sideburn and beard hair varies from candidate to candidate. Beard and sideburn transplants should do as well as scalp to scalp transplants.  Be sure your surgeon is an experienced hair transplant surgeon as the sequelae of the operation, especially if unsatisfactory, are permanent.