bbPress.org

The bb_users table uses an unsigned bigint for the user ID which allows numbers up to 18,446,744,073,709,551,615. How many people are there in the world?

http://dev.mysql.com/doc/refman/5.0/en/numeric-type-overview.html#id4784709

You would have more of an issue with hardware and bandwidth than you would with any kind of limit that is built into the software.