Easy: figure out how many simultaneous users a server can handle by having a beta program, then divide 2,000,000 by that number. That's how many servers you launch to handle 2 million users. So, if a server can handle 1000 users, you have 2,000 servers.
Bioware knows how many SWTOR installers they presold, so they have a good idea on the # of users they plan to have. Plus, they should have coded things so you can just slap in capacity and have scalability inherent in their server software design.
Also, I'm thinking they need to have unlimited free toon transfers (perhaps limited to 1 transfer a day) from high-pop to low-pop servers until s**t calms down. And those transfers need to be doable _from the bloody wait queue_!!