[JdeBP]

You should be first asking how many people are writing POSIX shell scripts because they need their shell scripts to work with Debian and Ubuntu.

* http://unix.stackexchange.com/a/250965/5132

* https://debian.org/doc/debian-policy/ch-files.html#s-scripts

* http://manpages.ubuntu.com/manpages/xenial/en/man1/checkbash...

* http://manpages.ubuntu.com/manpages/xenial/en/man1/posh.1.ht...

The answer is, of course, lots of them; as there was a massive project to do exactly this in Debian and Ubuntu some years ago.