There's probably less than 50 people that exclusively play on Tyranny who have more than one level 80.
Probably the same on DW or even less. You really can't count the lowbie minis or people leveling as it's hard to say if they will progress to end game content even with a merge. Actually, some of the lowbie players have been pvping at 19 for so long they consider that pvp now...
The majority of players play on all 3 servers: Set, Tyranny, and DW. People may pop over to Tyranny or DW on occasion, but the bulk of the population and most of their toons are on Set most of the time. And I can tell you that very few actually care about open world anymore, even the pvpers, so don't expect to see much from Tyranny or DW after the merge other than some alts fighting alts.
Doomsayer 2008