Currently, about as many as those who use Rust, although going by hype, Rust seems to be getting a lot more attention (i.e. larger derivative of the usage curve, perhaps?). If you look at usage on github, there's quite a bit of D code for such a niche language.
I've been doing Advent of Code in D, and I'm having a blast:
http://inversethought.com/hg/aoc/file/tip/2017/
I particularly enjoyed doing day 18, which required doing message-passing concurrency (or at least simulating it) and D's concurrency primitives fit the problem like a glove: