200 players in one map instance.
But not 200 visible player with interactions refreshing on real time. Most likely you see diminishing level of details, if anything besides complete fabrication or no data at all after a certain visibility range threshold.
In that case, let's assume that each player waves (/wave). I've done this before with >50 players and took a screenshot and we were all in sync. It's also possible to have 50 players in the same area fighting in real time, e.g., in a battleground. This was back in 2010 when I played ... So I am not sure if it's any fabrication at all, otherwise everyone would die, etc? Given players have health, take damage, get healed, etc.
But I don't believe WoW could do it now, the last time I played WoW, early last year, in a large raid, there would be spells cast from invisible people because the characters wouldn't even load.
But Britain bank with lots of people was always laggy. Especially if there wasn’t a server reboot in over a week.
It's certainly doable though.
In fact when they released WoW Classic they had a new technology they called layering where even if you were in the same realm as somebody you could be on a different layer and therefore not see them. People absolutely hated it and went crazy.
So that is why I asked him if he had played it since he said "at a certain point there isn't much difference to players between 50 people and 200 people within visible distance" which is just wrong.