If you spend any time in Taiwan, you'll come away with the impression that a great many people are surnamed Chen, Lin or Wang. This is something I touched on in my first book, in which I quoted a local idiom: "Chen, Lin, Li, Guo and Cai are half the people in the world." (Li is often spelled Lee, as in the case of a former president; Guo is often Kuo, and Cai is usually Tsai).
This webpage makes a fascinating comparison between the frequency of popular surnames in Taiwan with those in the USA:
Chen [the traditional version of character is shown right] covers 10.93% of the population... Smith, the most common family name in the United States, covers just 1.00 percent of the population there.
In Taiwan, the 10 most common family names cover half (50.22%) of the population. Covering the same percentage in the United States requires the top 1,742 names there. And covering the same percentage as Taiwan’s top 25 names (74.17%) requires America’s top 13,425 surnames.
Why is Taiwan dominated by a handful of family names? I'm not sure, but in South Korea the situation is even more extreme. There, the four most common surnames account for almost 70% of the population.