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I did a market sizing on "programmers" ( not just freelance ) about a year ago, and came up with around 20 million worldwide. If you take the statistic that 40% ( by 2020, or 35% today ) of US workforce is freelance and apply that it would be around 8 million freelance programmers worldwide.

Perhaps a really cool way of doing this sizing would to use "authorship attribution" techniques over webpage javascript to see how many "authors" there were -- tho that could likely be way off as well. Seems like a ridiculously involved way of calculating it tho.