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States with the most multimillionaires

The number of Americans worth $30 million or more grew by 6 percent last year, with the Dakotas and Florida seeing the fastest growth, according to a new report.

There are now 69,560 individuals in the U.S. worth $30 million or more—a group also known as ultra-high net worth individuals (UHNWI)—according to wealth research firm Wealth-X.

California had the largest number of ultra-high net worth residents, with 13,445. New York came in second with 9,530.

While California and New York added the largest number of new UHNW individuals last year, with a combined total of 1,470, in terms of percentage growth, oil powerhouse North Dakota and its neighbor South Dakota saw the most robust growth. North Dakota's rose 14.3 percent and South Dakota's jumped 13.3 percent from 2013 to 2014.

Florida's UHNW population grew by 10.3 percent in 2014.

The ultra-rich are highly concentrated in major cities. More than 65 percent are based in the top 30 U.S. cities. By city, New York had the most, with 8,655, followed by San Francisco with 5,460.

The number of "half-billionaires"—or those worth $500 million—grew faster than all other wealth tiers in terms of population size and wealth. There are now 2,031 half-billionaires and 571 billionaires.

Watch "Secret Lives of the Super Rich" on Tuesday, March 24, at 10 p.m. ET/PT.