I’m not aware of anyone running Caddy at any sort of scale for customers. From what I’ve seen, it tends to be only self-hosted people running it.

The overall trend in WP hosting has been away from shared and VPS hosting towards managed hosts, who provide a WordPress-specific environment and do things like upgrade WP and plugins for you. As a result, I can’t imagine there’s a huge percentage of sites using Caddy.

(That said, even a small percentage will be millions of sites, since the WP install base is massive.)