Curious, what is the max player limit or a good number for a multiplayer FPS that would involve each character piloting a vehicle. Should we aim for 8v8? I know Hawken only uses 6v6. Can we go 16v16?
What are the questions/considerations in terms of damage registration/stability?
I was about to start talking about MWO, but then I recognized the name, so I assume you’re asking for specs related specifically to Unreal Engine?
Anyway, to flesh this out a bit - The player number is directly based on how well you want your game to run.
FPS, server/client type, graphics, etc. All affect the amount of players you will have in the game.
The higher your graphical fidelity and complexity, the fewer players you’re going to want to have (cuts down on rendering of player models, and well as processing the actors/actions)
Are you going to have Permanent servers the game will be played on? will the “server” be one of the players?
How are you going to determine who hits who? will the decisions be made client side or server side?
The speed, style, complexity, framework, and overall design can change this.
Graphical scale can also be an issue.
Edit: Ah, went to your website, I see what you’re trying for.
I think it depends on the gameplay and the size of the packages you are sending each frame.
It’s dificult to predict exactly how a game will behave based only on it’s genre.
I would recommend testing when the base is done.
Both Zeriniel and Mielke are on the right track. High resolution player models will chew through your polygon budget rapidly, negatively impacting client performance. Sending tons of game data with every update will clog your bandwidth, increasing the processing time spent by the server just in handling updates and impacting server performance(and to some degree client performance as well).
Most games that allow high numbers of players to congregate in a small area (read that as “be rendered at the same time on a given client”) concentrate on reducing player model polycounts. This goes for weapons and other “accessories” that players carry, since they have to be rendered on the client as well. I’m focusing here because everyone wants their players to look awesome and this is an area that is relatively easy to control. 8k~10k polys should be good for player models (you could probably run higher, but I’m a cheapskate) and 1k~3k for weapons etc. Level design will play into this pretty heavily as well - the more “bling” you add the more load you place on each and every client.
If you have shaved the poly budget as low as you can and you’re still having issues, or if you can see that network latency is your bottleneck to begin with, then its time to start looking at the hard stuff. Start closely inspecting what data you’re sending over the network. I don’t think that UE4 sends crazy masses of information mainly because it doesn’t make sense to. I would guess that they use some sort of “dirty masking” to only send data that has changed, and only send necessary data, so unless you’ve replicated some giant hairy gorilla of a database by mistake this shouldn’t be an issue.
Disclaimer - I’m not a guru, just some things I’ve noticed over the last 20 years…
As you say, it really depends on how complicated the game and the characters are, and probably how well you can scale detail. The network code in UE4 is much more efficient than in UE3, so with that and faster CPUs player counts could be higher, but there is no fixed upper limit.
It might be interesting for someone to make a multiplayer game where players are just static mesh boxes and see how many players we can support. I may regret seeing that haha 
Hmm, do I detect a First-Box Team Shooter project in the works? Giant box levels with team-colored box players… I’ll bet we could get a pretty large pile of players in a game assuming the server had the bandwidth for it.