To answer this question meaningfully, we should include not just pure Bootcamp users, but those who run Windows using virtualization software. The total is
roughly 9-10% of the current ~100M Mac user base: ~2% who use Bootcamp directly, and maybe 7-8% who use Parallels/VMFusion/Virtual Box.
Details:
Parallells says >6M of their customers are using Windows on the Mac (
https://www.parallels.com/blogs/parallels-desktop-users/). VMWare says it has 500k customers (though those might not be all VMFusion), and then there's Virtual Box, which is free, and thus could have a decent no. of users for that reason. Adding in the 2M bootcamp users (2% of the ~100M Macs currently running), while at the same time recognizing that some people use both Parallels and Bootcamp, that gives roughly 10M people using Win-on-Mac, let's call it roughly 9-10%.