DistroWatch counts by clicks to the distro's page from what I remember. Its statistics represent the "interest", rather than install count.
With regard to Alpine Linux on Docker, the install count[0] of the base image should give an idea (over 1 billion!).
[0] https://hub.docker.com/_/alpine
Edit: clarity
I've been down this path before and I can say for sure two things -
1 - Distrowatch stats are both gamed and useless
2 - There's no easy way to see which distro is most popular on the whole.
Wow, that's impressive! However, when I check on Distrowatch it's only in the 30s. Is there a better statistic? Something more representative of actual amounts of installs on anything at all?