There were tons of people in Shadowmoon during WoD and it really wasn't that bad. The worst part was people getting pissed at you for being quicker at tagging rares and them raging on you.
I hope there won't be so much lag. It is over 10 years that WoW Startet and you can now rent resources for single hours. There is no need anymore to buy giant servers. It should be possible to do this..