There are millions of people who browse Wikipedia in any given month, but only 2 percent of them (roughly 1,400) are responsible for editing nearly 75 percent of the information on the entire website.
via College OTR: 73.4 Percent of All Wikipedia Edits Are Made By Roughly 1,400 People.
Now there's an interesting thought for you.
First, those statistics are old, there are many more regular editors on wikipedia today. Second, the so called "2% rule" is nothing new and is present on pretty much every web2.0 site, or any sort of system that involves user contribution. 90% of users are lurkers, another 8-9% are light content generators, and about 1-2% do most of the work. It follows a standard distribution bell curve.
Furthermore, this article is being very paranoid about the statistics. A small group of people may be responsible for most of the actual writing of articles, but that other 26.6% comes in the form of minor edits, corrections, fact-checking, and adding credible references. You will be extremely hard-pressed to find articles that were only written entirely by one person.