In fairness, Dark Phoenix does have quite a big cast:
Xavier, Jean, Cyclops, Storm, Beast, Nightcrawler, Quicksilver, Mystique, Magneto, Red Lotus, Selene, Chastain's alien leader, Jean's dad, and maybe others (the right-hand man to Chastain's alien, plus Dazzler, etc etc).
But this is Fox so many of those will be underdeveloped and marginalised. Everyone in Endgame felt like a fully rounded character with personality and motivation, i imagine very few in Dark Phoenix will feel like that. Bring on the MCU reboot...