Since gameservers are mostly single threaded they probably run 35~~ of them on a single instance ( c4.8xlarges have 36 vcores ), so if you have a good matchmaking algorithm you can fit 3500~~ players per instance, to reach 3.5M you need between 1000 and 2000 instances.

As for your last comment it's wrong, a game like Battlefield is probably more demanding compare to a battle royale type of game, a FPS like Battlefield runs at 60hz and is using a lot of physics everywhere at fast pace, battle royal games don't have that much action going on.