At the recent New Orleans Wizard World Comic-Con, Anthony and Joe Russo took part in the Captain America: Civil War panel which saw it remarked they had a list of 67 characters they were keeping track of for the Infinity War movies.
Now it turns out that "67" was just an exaggeration as the Russo's filled in the Collider, which also saw mention that "Thanos has a very large agenda" in Infinity War:
“We were being figurative when we said [67 characters] and people took it as literal but there’s a lot of characters. Infinity War is meant to be the culmination of the entire Marvel Cinematic Universe up to that point. It’s very ambitious in its scope, it wants to take everythign that you’ve seen before and coalesce into some kinda of climactic ending. It’s complicated , ambitious storytelling…We’re not talking about lead characters, just people that make an appearance.”
It's also learned when and where Infinity War will begin filming, and that the two-parter will be shooting at the same time:
“We start shooting both of them later this year in November and we’re primarily going to be based in Atlanta again. That’s where we’ll do out stage work and some exteriors. There will be locations from around the world involved as well. We actually relocate to Atlanta in late July to finish up pre-production there. We don’t come back to L.A. until the following June.”
“We’re shooting them concurrently, meaning that some days we’ll be shooting the first movie and some days we’ll be shooting the second movie. Just jumping back and forth. We won’t look like this next year.”
The Avengers: Infinity War Part 1 hits May 4, 2018, with Part 2 on May 3, 2019.