1200 players is already confirmed with Planetside 2. 30k player avatars are confirmed with Dual Universe. 7k player-battle with Eve Online. There is always "however", however. None of those offers the physical simulation on the level CIG are promising, modular ships divided into components, additional thousands of NPCs with AI etc.
- Planetside 2 has relatively simple physics, issues with lag compensation and failing collision detection. It does not detract from the fun, though.
- The Dual Universe demo has no interaction between avatars whatsoever, which reduces the complexity immensely. The only state to share is position of each avatar and no interactions means that lag does not matter.
- Eve Online is an action queue MMO like every early MMO. No physics to speak of, added time dilation etc., so it is not even a linear-time game. And of course it lags like crazy - performance tanks to "frames per minute".
Let's see..
Planetside 2 has an older netcode indeed, still the effort is amazing for its time (that was released in 2012.. yes back then SC was just barely started - if CRoberts had done his homework he'd had taken a long, detailed look at that game and how it was made as the general design is really close to SC) and yeah it lacks lag compensation / prediction, but still it's a reference milestone IMHO for what it achieved vs the competition (and even still today it's not so shabby.. not too many competitors). Oh and it uses a custom made engine.
Dual Universe is entirely designed around its netcode. So yeah the game design itself takes care of not creating bottlenecks in network traffic anywhere by avoiding data-heavy interactions. Another way to do that would be using a much slower kind of gameplay, e.g. World of Warships or Mechwarrior kind of speed, which really simplifies handing network lag and traffic. I mention that because that was a real possibility for Star Citizen if they used a more realistic flight model (= closer to realism modded KSP for instance). Bespoke engine too.
EvE netcode and architecture date back to 2004, it's vastly outdated here, and yeah that time dilation is a crutch. Everything runs synchronous with their backing SQL database, and if you do not know what that means, these databases have comparatively slow write operations (due to locks happening on data you are writing). Needless to say their database servers are absolute beasts, but yeah that's not cutting it if you have a thousand players spamming real time actions in the same place (that's where each player will need to get a hold of an exclusive lock on the database to write their action/command, then release it for the next player etc... hence the action queue and minutes per frame). So yeah EvE has never been a reference there, it's one for its space trading game design and economy BGS, best in class, still today I think (unless I've missed a newcomer). If one was to design the backing persistence today they'd use a fast-write, eventual consistency, high concurrency system (there are a few choices on the market - with things like Redis that can handle the real time front-end for the players). Not blaming CCP though as when they designed and released the game, nothing like this really existed. Custom engine again, starting to see a trend
