Saying that dialects can be roughly grouped into Eastern and Western hardly means there are
only two dialects in all of Japan. (And is the "one-half" Kyushu dialect? Does it only count as a "half" because one scholar considered it a subset of Western Japanese?)
It's not easy to give a precise answer (I'm not even going to attempt it because I'm not a linguist), but from a practical perspective of almost anyone listening to the differences, the number is going to be much, much higher than 2 1/2. Even
this diagram on the same Wikipedia page probably paints a more accurate picture.