Other common titles of wealth include multi-millionaire and billionaire. Since millionaire is a net worth of one million or higher it is inclusive of these larger wealth titles. Formally being a millionaire was a very exclusive club. Now billionaire is the exclusive club.
Forbes and Fortune magazines maintain lists of people based on their net worth and are generally considered an authority on the subject. According to Forbes' 17th annual list of the richest people published in 2003 there are 222 billionaires in the world. The number of millionaires would be much, much higher.