It is very difficult to find reliable information about that, partly because it is so difficult to define who is and who is not a buddhist in the West today. Some are devout buddhists, some mix different buddhist traditions, some even mix different religions, some are mainly buddhist but pick and choose their own understanding of what it means, some use buddhist methods and inspiration without calling themselves buddhists, and many people sympathize with buddhism and are maybe inspired by reading some popular books by famous buddhists like the Dalai Lama, without practicing anything from the buddhist teachings.
That said, we can see three major waves of buddhism coming to the West:
1. in the first half of the 20 century, the Theravada teachings totally dominated the scene, often mixed with theosophy and spiritualism
2. after WWII, and especially in the 50s and 60s, Zen became popular, among other things through the beat poets and the Japanese martial arts.
3. after 1959 and the subsequent exile of the major part of the great Tibetan masters, and especially the last two or three decades (one milestone was the Nobel’s Peace Prize of the Dalai Lama in 1989), the Tibetan teachings have more and more become the probably most popular and widespread form of buddhism, if you see the West as a whole.
These three traditions are still the three major players on the Western buddhist scene, even if a lot of other buddhist traditions have also had some success, like Chinese Ch’an (from which Zen originated), Chinese and Japanese Pure Land, Japanese Nichiren, and so on.