We've already seen huge ensemble casts in the MCU. The first Avengers was big at the time (though X-Men was probably bigger), and then Avengers: Age Of Ultron and Captain America: Civil War dialled things up massively with even more actors as Marvel refused to kill too many characters off. If you thought that was a lot, wait until you get a load of the two upcoming Avengers movies.
Already, Infinity War has an astronomical number of key characters, and Kevin Feige has been very clear - not least to Chris Evans - that there is so much story and so many characters to balance that Infinity War has to spill into Avengers 4 (even if it isn't titled Infinity War Part II any more).
By the sound of it, that would suggest that they're still not planning on getting rid of too many characters when Infinity War comes to an end (even with the threat of huge world-changing drama), so we can probably expect Avengers 4's cast-list to be just as packed.
Still though, with the secrecy over what will happen in Avengers: Infinite War and the possible fates of some characters unclear, we don't have firm confirmation that too many characters are back for its sequel. There's no talk of Thor or Doctor Strange yet, for instance, and we don't even know who Peter Dinklage is playing, let alone whether he'll be back.
There are some we know about though. Thanks to set photos, actor confirmations and the word of God (or Kevin Feige as he's more conventionally known) here's who we know for certain is involved so far in Avengers 4...