Some of the larger combats (5000+ ships) in EVE Online might do that.
(I single EVE out because other MMOs generally shard by user-cohort, so having that number of people on one shard is impossible. EVE, meanwhile, shards by location within the virtual world (each star system is a shard), so the entire player-base can "gather" on a single shard for a confrontation.)
I've never played EVE Online but from what I have read about it the engineering behind it seems interesting. Have any suggests for Dev Blog posts from their team?
(I single EVE out because other MMOs generally shard by user-cohort, so having that number of people on one shard is impossible. EVE, meanwhile, shards by location within the virtual world (each star system is a shard), so the entire player-base can "gather" on a single shard for a confrontation.)