Taekwondo is extremely popular. Since the early 1960s, taekwondo has flourished in the United States, and become one of the most common forms of self defense training, replacing many the Judo, Karate, and Kung Fu schools of past decades.
Canada has a large taekwondo following as well as the United Kingdom. Taekwondo is also very popular in Australia. It is difficult to say where it is "most popular" since you can find very dedicated, loyal, and skillful practitioners all around the world. Certainly Korea, Iran, Mexico, China, the US, Canada, and the UK are all very close in their popularity and practice of taekwondo.
The number of WTF members is currently 206.