Shankara is the figure the tradition organises itself around, and almost everything about him is harder than the received account.
His dates are conventionally given as 788 to 820, and those dates are disputed among scholars rather than established. His achievement is not in dispute. He argued Advaita Vedanta into the dominant philosophical position in Hinduism, wrote commentaries on the Brahma Sutras, the principal Upanishads and the Bhagavad Gita that remain the base texts of the school, and did it, by the traditional account, before he was thirty-three.
Tradition credits him with founding four mathas, monastic seats at the cardinal points of India (Sringeri in Karnataka, Dwarka in Gujarat, Jyotirmath in the Himalaya and Govardhana at Puri), each assigned a Veda and a great saying, each headed by a Shankaracharya. These are real institutions with real authority in the Smarta community, and their heads are among the most influential religious figures in India. Whether Shankara founded all four is a traditional claim rather than a documented one, and the succession at one of them has been the subject of protracted litigation.
The wider difficulty is that Smarta identity has soft edges by design. It is largely, though not exclusively, a Brahmin identity; it shades into Shaiva and Vaishnava practice at every point; and a great many Hindus whose practice is functionally Smarta have never encountered the word. That is why it does not appear in the demographic compilations that give figures for the other three. No credible estimate of the number of Smartas exists, and one should not be manufactured for the sake of symmetry.