I was there for beta and the opening of original WoW, and from my memory, the crowds were NOTHING compared to what have been on these WoW Classic servers. There was a lot of people, but it was no where in the ballpark compared to the thousands of people on each layer on these servers.
Now the one thing I have noticed is that "the crowd" is moving for the most part together in a level range. For instance, if you create a new character today during prime time on your server, you will not see tons of people there. You will see a healthy number of people, but you won't see shit tons of people around your level. Login to a level 20-30 character, and you will see a ton of people. In a week, they will be 30-40. There is a bubble of population moving through the level ranges.
That first week (hell even some in the second week) of Classic, I saw upwards of 30 people or more huddling around name spawns for quests waiting for it to spawn. When it would spawn, the mass of people would all swing once, and then someone would run away (the person who got credit). Before the next spawn, that person was replaced by a handful more people. It was insane, and I NEVER saw that back in origianl WoW Beta nor Release.