The number of millionaire households increased by 12.2 percent worldwide last year, according to a new report by Boston Consulting Group. According to the report, there were 12.5 million millionaire households. The United States continues to lead the world in millionaires, with 5.2 million millionaire households, followed by Japan with 1.5 million, China with 1.1 million and the United Kingdom with 570,000. Singapore leads the world in “millionaire density,” or the percentage of millionaires, with 15.5 percent of its population now millionaire households. Their combined wealth totaled  $47.4 trillion, up from $41.8 trillion in 2009.