I remember the crown frequently having 96+ per faction without any lag problems. Must have been clientside.
> (There is also eve-online where recently there was a 'fight' with 5000 players in the same location, but that is a different beast from FPSes, and the main logic of that game is single-threaded)
It was 6000, and it was terrible to be in. It took me 10 minutes to enter a station (normally takes a second) and I had to relog because I couldn't get out of the station. Relogging took another hour of black screens. It was pretty neat when I finally got out, but there wasn't much of a fight compared to previous encounters.
PS2 was for the most part an amazing game. The graphics were truly beautiful and the gameplay was very fun. Unfortunately, their shitty performance basically killed the game by making it unplayable for many users during the trademark huge battles.
I might be off on the numbers though.
I'd like to draw your attention to Planetside 2. A pretty recent FPS that was run a lot like WoW, with 3 'continents' and complete seamlessness on those continents. Firefights where 100 people were actively engaging each other were an almost constant experience back when I played it. I think there was a cap of 1000 players per continent. Those players could gather wherever they wanted. I'm pretty sure I was in some 300 player battles, and nothing but lag would prevent all 1000 people on a continent from going to the same base.
It has to be said that in the big battles (at around a 100) lag became an issue. I'm not sure whether this was clientside, serverside or both.
(There is also eve-online where recently there was a 'fight' with 5000 players in the same location, but that is a different beast from FPSes, and the main logic of that game is single-threaded)