68.

Film Avengers: Infinity War will have 68 characters according to The AV Club which seems like a lot, although the average MARVEL film usually has that many characters, taking into account day players and the like.

If these are all substantial roles, in other words the lead and supporting characters from previous MCU projects, my guess is that this is going to be more akin to a hyperlink drama or something in the realm of a quasi-disaster film, cross cutting between groups of characters affected by whatever's happening to Earth and galaxy, crosscutting between, for example, whatever's happening on Asgard to New York to Wakanda to Xandar (or wherever the Guardians of the Galaxy are) with everything eventually dovetailing for the sequel.

The really interesting question is how Phase 3 deals with their being a couple of stand alone films, including the Ant-Man sequel, in between.  Previously the films have worked as a kind of series, referencing each other with even the first Ant-Man film hinting towards Captain America: Civil War.  But it's possible that they'll be flashback's I suppose or the second Infinity War won't dovetail directly on from the end of the first film.

Anyway, none of this stops me from believing that Phase Four, due to end when I'm fifty, isn't going to be a version of Secret Wars, with a cunning way of dovetailing the MCU and the X-verse in a similar way as the recent comics event merged the 616 with its Ultimates cousin.