No credible figure exists for the number of Zen Buddhists worldwide, and the Japanese numbers that circulate are institutional rather than surveyed.
What can be stated are temple counts, which the schools report themselves. Soto has on the order of fourteen to fifteen thousand temples in Japan, the great majority affiliated to Soji-ji. Rinzai has around six thousand across fifteen branches with about forty training monasteries, more than half of the temples belonging to the Myoshin-ji branch. These are registrations, and the relation of a registered temple to a practising congregation is loose: for most Japanese households the tie to a temple is funerary and ancestral rather than meditative, a pattern often described, not kindly, as funeral Buddhism.
Korea has Seon, organised chiefly in the Jogye Order, and Vietnam has Thien. Both are substantial, and neither is well represented in the Western picture of Zen.
That Western picture is itself a specific historical product. Zen reached Europe and America largely through a small number of interpreters, D.T. Suzuki above all, who presented it in terms of direct experience, and it arrived detached from the monastic and ritual setting that carries it in Asia. Scholars have argued that the emphasis on experience owes as much to the modern encounter with the West as to the older tradition. Western Zen is now several generations deep, has its own transmitted teachers, ordains women on the same terms as men, and is markedly more lay, more egalitarian and more meditation-centred than anything in Japan.