Even relatively no name MMORPGs can sell 500K copies in the first month. That'd basically be suicide because people will say SWTOR can only pull in as many people as no name MMORPGs. While people complain about not being able to log on or queues, they will complain even more about how the game is dying if there isn't full servers. We're pretty much past the day where server condition can be so bad that it is basically unplayable. If you throw out servers that are just plain bad (i.e. stuff that crash even with nobody on and has nothing to do with how many are playing the game anyway), at worst you might have something is kind of laggy like say the RIFT world events where you can barely click on something before 200 people run over it. As much as people complain about how it sucks, the fact that there are 200 people beating up the first thing that spawned at any given place shows that such events are actually immensely popular. If it was actually frustrating to the point where it'd make you quit, then clearly you wouldn't have that many people participating (they'd have quit!)
Also the people who are likely to fall for these "OMG LIMITED RELEASE" are also going to be the most fickle, so that means a lot of those guys are going to be ragequit at the first sign of trouble. So, out of your initial 500K you should expect much higher attrition since the guys who are more indifferent about MMORPG problems are also not the guys who would just rush out and get this immediately.
If there was actually a limit it's likely to be something like 5 million, i.e. a number where if they hit then they already won as the #1 MMORPG of the market, and at that point they can obviously do whatever they want. It really wouldn't make sense to stop at say 2 million because that's a number where you'd want to try to go for the kill against WoW if you assume WoW is around 5 million NA+Europe. There is absolutely no reason to stop your momentum. Since they didn't actually say what the number is, I fully expect they'll just try to sell as many copies as possible and just say, "Turns out our servers are super awesome, who could've imagined that!"
I think Anarchy Online is the last MMORPG where the launch failed due to hardware issues (I heard the game was actually unplayable). Complaining about a MMORPG launch is basically the same as camping out in front of a Gamestop for a midnight release, and then complain it was cold outside. It's pretty obvious people who do this kind of thing are in it for the experience itself, because there is no sane reason to do so otherwise. It's a huge myth that MMORPG have rough launches. Yes life sucks if you happen to be on the server with faulty hardware, but that server would've died anyway even if it had normal population. Briarcliff in RIFT had a queue of 200 hours or something utterly ridiculous. Ask yourself why would you stick to a completely arbitary server (RIFT has no history, so we're not talking about a Sephiroth server for FF or Archimonde for WoW) and then wait there all day when there are 20 servers with no queue before you even created your character?