Every realm is still Medium or higher population, which means more than what would have made a realm Full in the old days, except Sul’Thraze, the Brazillian realm.
Pay no attention to the doomsaying on the forum; people here have been pushing hard for Classic to collapse long before it ever came out, for some reason.
That said, only play on a PvP realm if you really and actually want the opposing faction to be able to attack you at any time in most of the world, with no restrictions on level or numbers. “But I can’t reach a dungeon!” “But I can’t grind rep!” “But there are ten of them to only one of me, that’s not PvP!” More than enough people whining on that subject already, by far.