Not many, I'm sure. It's mainly the lady who sees fragments into the future, our cutie poison guy along with his close aides (like glasses guy) and the crazy maker guy who probably only realized he was j due to his eye skill and seeing that the weapon belongs to him. Plus...Our guy is just EXTREMELY bad at keeping himself low-key lol
I think in the earlier chapters the fragment girlie said she told that one woman too. Idk if she's the head of the burou (I can't spell) but the one who made Js sudden ranking public and claimed he's alive and they'll try finding him.
I think fragment girlie told her which is why J was so surprised that she could say for certain that the J on the ranking was him and that he's alive.

ok i might be slow at understanding things and because i have a little dislexia.
How many people know that mc is J?? like wasn't the point being him laying low, what happened now that J will return?? iam so sorry fo the dumb question