I know there are companies that produce games with 100 characters, but feedback that I've received about them is that they aren't very good. I can't say one way or another since I have not tried them.
I will say that when you invite 100 people to a murder mystery of this type, it's very difficult to actually include 100 distinct characters. From experience, not everyone wants to be a character anyway. There are those who will enjoy mingling and there's no reason why they cannot try and solve the crime too despite the fact that they are not a featured character.
The murder mystery Christmas game I feature on my site includes everything from decorating ideas to printable invitations. I've actually played these games and with the right crowd, they are a LOT of fun!