The estimate most often reproduced puts Vaishnavas at about four hundred million, roughly a third of all Hindus, in a database compilation for 2020. It should be handled with care. There are no census data on the traditions within Hinduism anywhere, India's census records religion, not sampradaya, so figures of this kind are modelled from surveys and from institutional affiliations rather than enumerated, and the boundaries they assume are firmer than the practice is.
Vaishnavism is strongest in Tamil Nadu, Andhra Pradesh, Karnataka, Gujarat, Bengal, Odisha, Uttar Pradesh and the Nepali hills, and its great temple complexes (Tirupati, Srirangam, Puri, Guruvayur), are among the most visited religious sites in the world.
It is also the form of Hinduism most visible outside India, for a specific reason. The Swaminarayan movement built large temples across Britain, North America, East Africa and the Gulf with the Gujarati diaspora. ISKCON, founded in New York in 1966 by a Gaudiya monk in his seventies, propagated a Vaishnava practice deliberately to non-Indians, and is now both a convert movement and, in several countries, the institutional Hinduism available to Indian migrants who had no temple of their own.
Both are Vaishnava, and both are frequently mistaken by outside observers for Hinduism as such. That is worth naming plainly: a great deal of what the English-speaking world knows about Hinduism it learned from one or two devotional lineages that chose, for their own reasons, to go out and teach it.