Maximum number of users on a forum

[emailapphost]

Is there a theoretical maximum number of users on a vBulletin forum?

Is there some limitation with databases etc?

How would you set up a vBulletin forum with say 16 million, or 50 million users?
(Say for example that a government entity would like to set up and pre populate with one account for every school pupil in China.)

[bananalive]

There are vbulletin forums with 8million+ users (eg. zenhex.com)