How many players are allowed to be at an UE game server at once? Unlimited?

The question is up there. Is game limited to 8 players or you can get it unlimited amout?

From what I want it is almost unlimited in theory, but without a custom server it is in practice on par with other modern engines and tops on around 64 players.

Planetside 2 managed to hold up to 3k at once. Still enough for my game.

AFAIK - There is no specific limit, only limited by your games ability to handle network traffic - e.g. how much data is replicated.

How about data that unreal engine auto-sends? How much playeres it can support?

Yes, but with a MMO style backend and much less players than that in the same battle.