In Eve online the largest battle had ~6500 players concurrently. Eve has a system called time dilation to help handle large fights. During a normal battle, ever

In Eve online the largest battle had ~6500 players concurrently. Eve has a system called time dilation to help handle large fights. During a normal battle, every player's actions are processed by the server, and the state is sent to all players at least once every second. When there are too many players and actions for this to happen, the servers overall tick rate slows down. So for this fight time dilation was down to 10s per tick, everything takes 10s as long, a normal weapon that fires every 15 seconds only fires once every 150s. Even with this 10x slowdown of actions, and a game like eve where relatively few actions are taken by each player there is a huge amount of lag, the server and clients don't always agree on the game state, weapons not activating, commands not being recognized. The underling issue here is the information needed scales exponentially with the number of players interacting. Supporting 200 players means 400x as much information as 10 players instead of 20x as much. So specifically for WOW, there are problems with more than 80 or so players at once in PVP. If you do things like large scale open world PVP getting desyncs, disconnects, heavy lag is common. Also for major cities and other high density areas most servers have 4 or more "phases" where you can be in the same place in the game and not see other people not part of your phase. So you could have 50 people in a guild all together, 200 people in local, but most of the other 150 in another phase.