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There are 21,951,000 millionaires in the United States, according to the 2021 Global Wealth Report. It’s a pretty safe bet that not all of them were born rich. In fact, the number of millionaires increased “substantially” between 2020 and 2021, largely due to housing price increases and stock market gains. Not all these millionaires are showing off their wealth. Some live below — or well below…