67!!!
The Russo brothers are claimed to have a chart showing 67 characters intended for Avengers: Infinity War.

Some days being a fan means headaches, and I've got one trying to think up all the heroes in the Marvel cinematic Universe, current and forecast......and the most I can reasonably come up with just skirts over about 40.
Obviously, I'm missing some...

Have to think we'll see some more cosmic characters by then. Honestly had hoped they would've done Ronan better and made him more heroic in later films to help fight Thanos, kind of like the comics did with Annihilation and everything. Seemed wasteful. Warlock...Phyla-Vell...Moondragon...