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)