10 Things You Need To Know About Avengers: Infinity War
6. It May Feature Up To 70 Characters
The Russo Brothers brought fanboy excitement and intrigue to a fever pitch when they made the off-hand comment that the film's production office has a board with "like 67 characters" on it, causing many fans to immediately assume the film would feature exactly that amount.
The brothers later clarified that this wasn't a hard-and-fast number, but merely the significant existing MCU characters who could appear, not discounting those they choose to omit and other comic book heroes they ultimately decided to include in the film.
Considering that Civil War alone has around 30 characters of note, it's not a huge leap to conclude that a film also encompassing the Guardians of the Galaxy among others could end up around the 60 or 70 mark.