Nobody can count Sufis, and the reason is structural rather than a failure of data collection: affiliation to an order is not exclusive, is often private, and does not replace the ordinary religious identity of the person concerned. A Muslim in Senegal or Sindh may be Sunni, Maliki or Hanafi, and Qadiri or Tijani, all at once, and would not describe the last as a denomination.
Where Sufi orders remain socially dominant, they are very dominant indeed. In Senegal the Mouride and Tijani orders organise a large share of national religious life, and the Mouride city of Touba is a religious and economic centre in its own right. Across North and West Africa, Sudan, Pakistan, India, Turkey, Central Asia and Indonesia, orders and shrines are woven into ordinary practice. Historically the orders were also the main agents of Islam's spread into sub-Saharan Africa, South Asia and the Malay world, which is why the map of Sufi influence and the map of Muslim expansion are largely the same map.
The twentieth century was hard on the institutions. Turkey banned the orders and closed the lodges in 1925 under the secularising republic, and the Mevlevi ceremony survives there partly as sanctioned cultural performance. Soviet policy suppressed the orders in Central Asia and the Caucasus, where they nonetheless persisted underground. Modernist and reformist Muslims criticised Sufism as superstition and as a cause of political weakness.
In the West, Sufism has had a second and quite separate life, detached from Islamic law and presented as universal mysticism, Rumi is among the best-selling poets in the English-speaking world, generally in versions made from other translations rather than from Persian, and generally with the Quranic content removed. Sufi teachers inside the tradition regard that development with something between amusement and alarm.