----
I have cited this before, on various threads. It's taken from the World Christian Encylopedia, published by Oxford University Press. (http://www.ywam.org/books/wce.htm). The Web site says it was published 'late December 2000.' I assume this is the most recent revision of the book, since I've seen it cited in other articles dating back to at least as early as 1996.
I don't personally own the book, but I would be interested to hear what qualifications they used to distinguish separate denominations. I would venture to guess that it had to do with particular doctrinal differences, of small or great significance (e.g., whether one believed in post-trib, pre-trib; whether one believed in the real presence, the necessity or existence or efficacy of Sacraments; the necessity of ordained ministers; etc.) Although I would certainly argue that there are many different denominations that are for the most part identical in doctrinal beliefs, I find it very difficult to believe that the only spot of contention that separates them is whether they believe it's OK to use snare drums in a worship song.
I would venture that of those 28000 denominations only a max of 200 have any kind of significant outside study done of them. I would also venture that the doctrinal differences are not what they are using to distinguish, but names. If there's a name, there's another denomination.
A recent book that I looked at (can't even remember the name) listed denominations and one of their qualifications was that the "denomination" must have over 5000 adherents (really small number really). They had about 120 or so listed, including some quasi-Christian groups - Quakers, etc. and some outright cults - Muslim, JW.