Theravada accounts for roughly a third of the world's Buddhists. An estimate widely reproduced in the reference literature puts the branch at about 36 per cent of all Buddhists as of 2010, against a Pew count of some 320 million Buddhists worldwide for 2020. The two figures come from different exercises, and multiplying one by the other yields an impression rather than a number.
It is the majority religion of Sri Lanka, Myanmar, Thailand, Cambodia and Laos, with substantial communities in Bangladesh, Vietnam, southern China, Nepal and among the Tai peoples, and it has been carried to Europe, North America and Australia by migration and by conversion alike.
Two modern developments have changed it more than any doctrinal argument. The first is the vipassana movement. Insight meditation, until the modern period the specialism of a small monastic minority, was reorganised into a portable technique taught to laypeople in courses of fixed length, largely through Burmese teachers from the late nineteenth century onward. That is the single largest reason meditation is what most Westerners now take Buddhism principally to be.
The second is entanglement with nationalism. In Sri Lanka and in Myanmar the identification of the religion with the nation has drawn monks into politics and, at intervals, into the defence of communal violence, against Tamils in the first case, against the Rohingya in the second. The tradition contains its own strongest objection to this, and monks have made it.