That number results from using what should be the correct definition of "dwarf planet" as a subclass of planets, specifically referring to small planets that do not dominate their orbits but have attained the crucial state of hydrostatic equilibrium and therefore have the geophysical properties that make it appropriate to designate them as planets.
In order of distance from the Sun, the planets are: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, MakeMake, and Eris.
Being that Haumea is so oddly shaped, I was initially surprised at its being granted dwarf planet status at all. However, according to Dr. Alan Stern, premier expert on Pluto and the Kuiper Belt, hydrostatic equilibrium doesn't mean an object has to actually be spherical. "It just means in fluid equilibrium, which can and does include centripital forces...EL61 is certainly massive enough to be in hydrostatic equilibrium where (it) is spherical. That it is not is just a detail due to the high spin rate, but spherical is not the real central issue in the dwarf planet or planetary definition, it's hydrostatic equilibrium." It turns out EL61's odd shape is due to its being elongated due to a very high spin (rotation) rate.
Discoverer Dr. Mike Brown, in responding to a comment on his blog, notes that there are likely many additional Kuiper Belt Objects in hydrostatic equilibrium that should be given dwarf planet status. This is undoubtedly true. In some cases, these objects' tremendous distance from us combined with their small sizes makes it difficult for us to determine whether any specific individual one has attained hydrostatic equilibrium.
What this means is that for a time, either years or decades, we will have to live with a degree of uncertainty about these objects. For now, it is useful to refer to them as "dwarf planet candidates" until we have sophisticated enough tools to determine whether they actually meet the criteria for consideration as dwarf planets.
Daniel Fischer, a supporter of the IAU planet definition, claims that the fact that we will likely end up having several hundred dwarf planets in the Kuiper Belt "devalues" the term planet because there will be so many of them. What exactly does he mean by "devalue?" Does being "special" mean the number has to be limited and the occurrence of a phenomenon scarce? Until the 1920s, astronomers believed there was only one galaxy, the Milky Way. Now we know there are billions. Does that diminish the significance of the term galaxy? Does the fact that there are billions of stars diminish the value of the word "star?" Fischer's logic is hard to understand because it is so subjective.
In the early days of civilization, humans believed the Earth to be the center of everything. Over subsequent centuries, we came to realize we are one planet in one solar system in one galaxy in a universe with a multitude of galaxies, solar systems, and planets. We're not inherently "special"; we're one planet of many. Similarly, the term planet does not somehow lose its value because we have discovered that instead of there being nine in existence, there are hundreds, both in our solar system and in others.
Fischer also argues the need for a fixed number of planets in our solar system. This too is unnecessary. We all grew up in a world with a fixed number of nine planets; the real change we must acknowledge is not that that number is now reduced to eight; it is that that number is no longer going to remain fixed. Even looking only at our own solar system, the rapidity of new discoveries makes it clear that the number of planets orbiting our sun--or rather, the number of which we are aware--will continue to be in flux for a long time. There is no logical reason why textbooks and teachers cannot explain this fact. Not only will doing this save the need to publish new textbook editions every time another planet or dwarf planet is found; it will also will bring home the excitement that discovery is ongoing and that we do not need to artificially set the number of planets orbiting around our sun at any particular number.
Brown states that his main concern is distinguishing the eight gravitationally dominant objects in our solar system from those that are not gravitationally dominant. That can in fact be accomplished while still acknowledging that dwarf planets are planets due to their differentiation and geophysical characteristics.
In 2000, Stern and Dr. Hal Levison published a paper explaining the existence and distinction of two classes of planets. That article can be found here: http://www.boulder.swri.edu/~hal/planet_