I do not particularly aim for more than 1000 players concurrently online but it would eventually be nice.
What I have read is that single server at least in CS model could support around 2000 - 6000 players - so in that sense, I think you are aiming kinda big from the start. However, the number of supported players also depends on the type of the game you are making (and how you program it). Such as fast-phased FPS with same CS model might scale only to hundred. I would first try my best with CS model (because it is easiest to implement) and use possible compensatory techniques to make it scale better. After that, modifying it to client-multi-server architecture and implementing interest management technique with regions should still be possible. Thus, I bet you enjoy the moment when you realize that you have a working MOG over CS model - even if it can't support "millions" from the start
Considering the interest management and regions handle by different servers, I think at least following articles gives good details / more depth overview:
Peer-to-peer architectures for massively multiplayer online games: A Survey
http://dl.acm.org/citation.cfm?id=2522968.2522977
Note: Despite the name of the article it gives good overview of interest management including relevant techniques to make MOGs work
A Communication Architecture for Massive Multiplayer Games
http://dl.acm.org/citation.cfm?id=566503
A Distributed Architecture for MMORPG
http://dl.acm.org/citation.cfm?id=1230040.1230067