How many people actually know about Ranma's curse?

[Shadowbakasama]

In Fannon everyone at Furinkan except Kuno knows about the curse and Ranma has to deal with being splashed in the locker room. But we don't see Ranma explain the curse that many times in the series, and we don't see Ranma interact with that many people outside the regular cast.
I think there were a couple scenes where Furinkan students seems to know that Ranma-chan is Ranma, the Romeo & Julliet play, and the basketbal game with Ms. Hinako, and I think those were anime.

[Atlan]

We never see him telling Dr Tofu, but he figured it out by the Ranma/Mousse fight.
When Tsubassa came to town, and had a food selling competition with Ranma, no one would by from girl Ranma. His(her) friends said that everyone knew that (s)he was really a boy.
Principle Kuno was fooled at first, but also figured it out off camera (or what ever you want to call it).
All these references are from the manga.
By the time that Ukyo was on the secne, Ranma had to wear a discuise to fool Furinkan with his female form.

[Drawde]

I can't find all my manga volumes to check, but I think most of the school knew after the first fight with Mousse. I believe Ranma, still under the effect of the cat's tongue point, was swimming at school when Shampoo showed up to tell him about the phoenix pill. I think one of the other students even asked about Ranma changing back. Viz volume four.
In the anime, the writers hid the curse from everyone. In order for one of the jokes to work during the cheerleading fight, they had Genma show the spectators the curse.

[mondu_the_fat]

In the manga, every one who knew them (except the Kunos), plus almost everyone else, quite early on.
"... Kuno... and only him" (onlookers, but I can't pinpoint which book right now)
and
"Ranma! King of the crossdressers!" (the drama club, who had not met Ranma before, Romeo and Juliet arc).
http://ranmahentai.ranma.ws/RanmaManga/ ... 08-072.jpg
(the details of the curse may have been missed by outsiders, but not those who knew him personally)
and, yes, drawde got it right: after the Mousse battle, the students were already asking when Ranma would change back (during the fight, they believed it was a disguise that could melt).
http://ranmahentai.ranma.ws/RanmaManga/ ... 05-160.jpg
Later on in the stories, only those from outside Nerima have demonstrated ignorance of Ranma's curse (Pink and Link, Herb).

[indolence]

"... Kuno... and only him" (onlookers, but I can't pinpoint which book right now)

That'd be Volume 10, Tsubasa's arc.

[antimatterenergy]

Ranma hasn't really told anyone about his curse but he also made very little attempt to hide his curse so just about everyone knows about it somewhat. Those who haven't seen it in person probably think he's a cross dresser like the drama club from the rumors, or that they're is a male and female named Ranma, and a lot of people probably think that it's some kind of trick. I'd guess that most people who didn't see it in person would not believe that Ranma changes and would have a hard time getting someone else to believe it unless they also saw it. Those that know for sure are his classmates (he's changed sexes many times during class or around the school) and the people he hangs around with.