What the source for the quote that there's over 20,000 Christian denominations?

I have heard that there is estimated to be more than 20,000 Christian denominations (protestant). Where does this come from? Is there a source where I can get more specific information on this?

Source of information:

World Christian Encyclopedia : A Comparative Survey of Churches and Religions in the Modern World (2 Volume Set)
By David B. Barrett

Available for purchase here.

Another interesting site for religious statistics is here.

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