For a single server using a server made in DBPro, yes. However, one thing you have to concider is that MMOs are broken down into multiple servers.
No I don't mean like "oh I play on the Jupiter server" and "I play on the Saturn server" Those server names are actually the names of entire clusters of servers. Typically, an individual server within a cluster rarely goes over 100-200. Upon reaching that limit, many MMOs will actually instance the server. Which is much like instancing an object. It basically creates a duplicate. You see this on Everquest2 a lot "Antonica", "Antonica 2", "Antonica 3" etc. Also on City of Heroes. Especially during peak hours.
Lastly, you can theoretically run multiple servers on a single computer given enough ram, bandwidth and cpu power.
One of the major problems with making an MMO on your own is that even the best programmer who has reached a state of ultimate enlightenment can only write about 25-50 lines of useable, debugged code per hour. An MMO is likely to contain hundreds of thousands or even millions of lines of code.
And of course, as anyone who has experience can tell you, the more people programming one thing, the exponentially longer it takes.