Evans Data’s Developer Marketing 2014 survey of 450 software developers showed that 19.3% (or approximately 3.5 million) developers worldwide are women, compared to the years between 2003 and 2009, when the percentage of female developers was in single digits.
Record number of women in software development, survey says - SD Times: Software Development News
Of note, if you do the math (thanks to @barton808 for correcting my bone-headed thinking) you get 17.5m developers world-wide.