There's too many undefined or barely defined terms there. MMO is such a loaded term. Without knowing how much CPU processing a user needs, how much backend storage a user needs, etc... It's impossible to say.
I mean just at a bare minimum assuming the hardware can support it, you would need 8 front end servers just to support 500,000 connections. (TCP max of 65,536 ports per VPS rounded down to 65k.) I assume you've read about the C10k problem (The difficulty in scaling a single server to 10,000 connections, and 8 front end servers assumes you're at 6 times that rate.) If not take a look at http://www.kegel.com/c10k.html
However, the metric I've used when it comes to MMO buildout was to assume 1 'CPU' per 100 connected users. This worked out to be a good average from 2001-2005 but I haven't developed an MMO since. This 1 CPU basically just encapsulates the cost of gameplay servers, front end servers, database servers, content distribution servers etc... It worked whether it was a blade server with 10 CPUs or old dual Pentium Pros. Of course as an added example those build-outs usually assumed one full time Network Operations person per 5,000 connected users. (Both obviously scaling with the peak-tie ratio.)
I'd advise getting to your scalability tests sooner than later because I think it's going to wake you up to some problems in your design. Get 50,000 people on 1/10th the hardware you expect to run in production and see where the seems come apart. (Ideally do that 12-18 months before you expect to ship.)