I did some looking into Wikipedia - according to one research paper the average Wikipedia article has 26 links to other Wikipedia pages.
The Wikimedia CMS linking algorithm is automatic from memory. However it is applied judiciously - not every occurrence of a term is a link, and in some cases it is not the first occurrence on a page that is the link. Not exactly sure how the software handles this or if it can be manually edited.
I'm thinking about this for our site - my preference would be to roll out a program of internal linking over 6 months for example.