I don't keep count but over the years I'd say the number is in the several hundreds, including one-time volunteers. We do have a core set of what we call "editors", who are volunteers in charge of managing individual productions, along with a solid amount of repeat contributors who work on ebooks as they feel like it.

I think people participate because it feels good to make something of quality, and then give it away. If you're passionate about literature, then you get to read a lot of great stuff while you're doing it, too.

To tack on, how could one join as a volunteer?

See here: https://standardebooks.org/contribute

Our number one need is ebook producers. If you're passingly familiar in a command line environment and have basic HTML/CSS knowledge, you can produce an ebook!