Countries with the most multimillionaires

The rich are getting richer—but at a slower pace.

A new report from London real estate firm Knight Frank says the number of multimillionaires in the world will surge by more than 25 percent over the next decade. But that's much slower growth than the past decade, when the pace of multimillionaire growth topped 50 percent.

Skyline, Ho Chi Minh City, Vietnam
John Harper | Photodisc | Getty Images

The report, conducted with WealthInsight, found that there are now 167,000 people worldwide worth $30 million or more, up 59 percent since 2003. There are 37,104 people worth $100 million or more (up 62 percent since 2003) and 1,682 billionaires (up 82 percent over the same period).

In 2023, the report predicts there will be 215,113 people in the world worth $30 million or more, up 29 percent. The ranks of those worth $100 million or more will grow 31 percent, to 48,473, while the number of billionaires will grow 38 percent, to 2,315.

(Read more: 10 most expensive real estate markets)

"This is a slower pace of growth than over the past 10 years, although much faster than the 8 percent growth seen between 2007 and 2013," the report said.

Richest towns in America
Richest towns in America

Asia and emerging markets will drive much of the growth. The report said the country expected to see the fastest growth in millionaires is Vietnam, with a projected growth rate for $30 million-plus residents of 166 percent.

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Indonesia ranked second, with an expected growth rate of 144 percent, while Ivory Coast ranked third, at 116 percent.

Granted, all of those countries are growing off tiny bases, with Vietnam expected to have only 293 people worth $30 million or more by 2023.

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The U.S. is expected to have the most multimillionaires by far in 2023, with 47,468 people worth $30 million or more—up 21 percent. The U.S. will account for more than one-fifth of the global total of the $30 million-plus population.

—By CNBC's Robert Frank. Follow him on Twitter @robtfrank.