There are a number of Classic Era PvP clusters. I’ll repost this snip from a Blue detailing which realms belong to which clusters:
Whitemane (+9other realms) comprise the NA PvP West
Benediction (+4other realms) comprise the NA PvP East 1
Earthfury (+7other realms) comprise the NA PvP East 2
Bloodsail Buckaneers is its own RP-PvP realm (no connections)
I myself play Horde in the PvP East 1 cluster. It’s certainly a smaller community than the free transfer realms but it’s the native home of my guildies so I’ll happily be their mascot on the forums. Above all though, I would stress taking your time with choosing where you want to play in order to find the spot that best fits the experience you want from the game.