I can speak for EU. I think it's fairly quiet ATM due to the PvP changes where most 20s left to twink in classic. From bg pop experience it defo seems alliance is the more populated as they usually have less players when BG pops and rarely get filled.
I made a toon on defias brotherhood as heard RP realms were more populated with twinks. From my few weeks playing I can neither agree or disagree the game just seems generally quiet in the major cities outwith dragonflight.
I hope for some kind of positive change with the new expansion be that increase of f2p level or separate queues from 29s but we will see.