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.

iirc every intel cpu runs minix on its management engine. Might be a contender.