Wikis - how many are there?

RationalWiki is my newest favorite.

https://rationalwiki.org/wiki/Main_Page

Companies may have internal wikis. We have one for each of the sub-departments under Marketing—Advertising, Content, and Social Media. I'm not sure if the other departments have such wikis although they might.

There are wikis for all sorts of fandoms (like for Star Trek there are separate wikis for the TV/films, the novels, and at least 2 more I know of for fan fiction). No doubt you can find a wiki for most TV or film properties if they're at all popular or new (for example, it's easy to find a wiki for Charmed, but not for Mr. Ed).

Probably millions of them out on the interwebz by now.

It looks very interesting, I put it in the sites I want to visit.