I am also curious as to how the figure was arrived at.
I’m thinking, well, you could count the Seventh-Day Adventists as two, well, actually three because though David Koresh has gone to his eternal reward, the Branch Davidians are still around, and the BD is a splinter from the Adventists. But there are two major branches of Adventists, one of which is relatively orthodox (I said “relatively”) the other verges on being a cult.
Then there are “house churches” with congregations of 10-20 or so people- we had quite a few of them in my hometown of South Bend, IN. They tend to be Pentecostal in flavor, but if a few of them are in any way affiliated with one another, they could constitute a denomination.
Nearly all of the denominations have had splits within the denomination, and then splits within the splits. So while one person might think of “Baptist” as a denomination, another would see Northern Baptist, Southern Baptist, backwoods snake-handling Baptist, etc., and consider each group a separate denomination.
Penecostals? Plenty of independent mega-churches large enough to be a denomination unto themselves. Then there’s the Vinyard Movement and the Toronto Airport movement (actually both part of the same movement) so you’ve got a new denomination there… Protestant congregations are also notorious for breaking up into smaller congregations, often over doctrinal issues. So if a splinter congregation gets large enough that a second church that agrees with it doctrinally is started, bing, new denomination.
Conceiveably, you could define a group that consists of maybe a hundred or so people as a denomination, see house churches above.