How short can you wear your hair in back?

[Nitro]

Hello all, first time caller long time listener! I'm sure this question has been asked many times before but I couldn't find anything in the archives.

I wanted to know how short(clipper #) you can wear your hair in back without making the donor scar visible? There's a guy in my office that I know had a hair transplant because I can see the line from the donor scar in the back of his head. He has blonde hair and chooses to wear it really short. The transplant itself looks great and to be honest if he didn't wear it so short in back you wouldn't even know. Good for him that he doesn't care but I would choose to try and hide it.

If anyone can speak from experience it would be helpful info to know. This is one of my biggest concerns when considering a HT. I really want the flexibility of wearing my hair at different lengths and styles.