The numbers here are contested in a way worth stating plainly, because the gap between the claim and the estimate is a factor of two to four.
Soka Gakkai reports a following of some 8.27 million households in Japan. The academic estimate, by Levi McLaughlin in 2018, is two to four million adherents; a national broadcaster's survey in 1996 found around four million individuals, about 3.2 per cent of the population. Both should be given. Household registrations and individual self-identification are not measuring the same thing, and an organisation counting its own rolls has no particular incentive to prune them.
Soka Gakkai International, founded in 1975, is by its own account present in more than a hundred and ninety countries and territories, and is the form in which most people outside Japan encounter Nichiren Buddhism at all. Its membership is unusually diverse by the standards of convert Buddhism in the West, drawing substantially from populations the meditation-centred traditions have not reached.
Nichiren Shoshu reports something over eight hundred thousand members and around seven hundred temples in Japan, and that figure too is its own. Nichiren Shu and the other traditional schools continue on a smaller scale, and the twentieth-century lay movements Rissho Kosei Kai and Reiyukai, both built on the Lotus Sutra, are substantial in their own right.
The exclusivism has softened unevenly. Soka Gakkai now presents itself in terms of peace, culture and education, and has formally abandoned the confrontational conversion practice of its early decades.