^ Second that. There is no reason to have this arbitrary limit of 4. Coding wise, it is of little difference.

Why is the limit for human players so low?

Not speaking out of any knowledge, but I'm guessing the limit is there because waiting for more than 3 other players to take their turns makes the game too slow. Even with three, you could easily be talking 30min or more between turns. Maybe a warning rather than a hard cap would be a better approach though, so those that are willing to deal with that kind of pacing can.

I think 4 might be too low for me too. If there are going to be simultaneous turns then 8 seems much better!

Because then it wouldn't be a TURN-based game.

Not being snide, but once you get out of the lockstep idea of having only one side be allowed to perform an action, you start to introduce all sorts of problems of synchronization and "who gets to do what first" issues. That doesn't even bring up the issues of automatic moves.

Frankly, once you've gotten out of the locked turn concept, you might as well move directly to Real Time, because the problem is actually easier if you go away from player-defined turns, and step directly into automatic milisecond turns (which is how RTS games actually work).

Overall, I think the best compromise here to accommodate large numbers of human players would be to strictly enforce turn time limits. I'd say 5 minutes, max.  That encourages good automation and management by players, and kicks the feet out from under the micro-managers.

At best, for "simultaneous" play (that is, allowing other players to take actions out-of-turn), I'd vote for allowing only NON-SHIP actions; e.g. managing your planets. I'd probably not even want to allow initiation of trades (i.e. only the "active" turn player can initiate deals).

I think it was pointed out above that there should be no technical reason to not allow a large number of humans.  Certainly, it should be constrained only by the performance of the least-capable system attached, primarily in terms of memory available.

It looks to be only of a User Experience challenge.

And, speaking as someone who has access to some pretty ridiculously high powered systems, it would be really nice if we could run everything in a "1 host/multi-guest" setup, where the guests handled the UI and graphics load, but the backend computation of actual moves was handled (Mostly) by a dedicated box.

I mean, right now, there's a 12 core/24 thread box with 48GB of ram sitting next to my feet, idle.  Sure would be nice to run it as a server and let lots of less-advantaged people hook up to it for some sort of epic multiverse shebang.

Simultaneous turns generally plays far smoother and faster for multiplayer.  The principle downside is gaminess involving clicking faster for moving a stack of units and gaining an advantage that way.  There's ways around this, though - like only executing unit moves at the end of turns, or stopping the game to allow reaction moves in response to a threatening move by another player.

I guess an option to turn it off or on for multiplayer might be fine. But if it came down to either individual turns or simultaneous turns then I would prefer individual turns.