A current gen WoW realm generally has around 5-35k characters on it total (only level 10 and above that have been active within 30 days). I don't think blizzard publically says how high their servers can go for concurrent connections, but I'd imagine it's probably around 5k. Most WoW realms don't have close to that on at one time.

The answer to server stress problems is more servers, not trying to max them out higher. If I remember correctly I think XI servers did around 1-2k on average?