Shia Muslims of all branches are usually estimated at ten to fifteen per cent of the world's Muslims, with Twelvers the large majority of those. The ranges are wide because several of the countries concerned do not collect or publish religious affiliation reliably, and because in some the question is politically charged.
Twelvers form the majority in Iran, Iraq, Azerbaijan and Bahrain, and very large minorities in Lebanon, Pakistan, India, Afghanistan, Kuwait, Turkey and the eastern province of Saudi Arabia. Iran has been Twelver by state policy since the Safavid conversion from 1501, which is a reminder that the present religious map of the region was made comparatively recently.
Two facts dominate the present. The first is that Twelver Shiism is at the centre of regional geopolitics in a way that has little to do with theology: the Iranian revolution of 1979, the Iran-Iraq war, the removal of Iraq's Sunni-dominated government in 2003, and the armed movements Iran supports in Lebanon, Iraq, Syria and Yemen have all been read through a sectarian frame by governments on both sides, and that framing has hardened communal identity in populations that had not previously thought in those terms.
The second is persecution. Shia communities have been targeted by sectarian violence in Pakistan, Afghanistan and Iraq; shrines and mosques have been bombed; and the Islamic State declared Shia Muslims apostates and killed them in large numbers. In the opposite direction, human rights bodies including United Nations rapporteurs have documented the Iranian state's treatment of its own religious minorities and dissenters.
Neither of these belongs to the theology. Both determine what being Twelver Shia currently means in most of the places Twelvers live.