How many people can write clean, efficient and maintainable JavaScript?

And Erlang is not the only sane option for this kinds of problems, there is also Go. And to a lesser degree you can do the same in many other languages given the right libraries and careful thinking, it takes more effort than with Erlang or Go, but almost anything beats JavaScript in both performance and code clarity (both at the 'low' code-readability level, and at the high 'project organization and design' level).

Depends on where you are. In Stockholm I would not be surprised if more people can write decent Erlang compared to the people who can write decent JavaScript.