The Dax symbiont in Star Trek: Deep Space Nine had nine official Trill hosts across the centuries, from Lela to Ezri Dax (Nicole de Boer). First introduced in Star Trek: The Next Generation, the Trill were a humanoid species of whom a select few were chosen to join with a symbiont. The resultant symbiotic bond between the symbiont and its host meant that each host retained the memories of their predecessors. This allowed Lieutenant Jadzia Dax (Terry Farrell) to continue her friendship with Captain Benjamin Sisko (Avery Brooks) to whom her former host Curzon Dax had been a mentor.
The Trill process of becoming a symbiont host was hugely competitive, with only 300 hosts selected each year from around a thousand applicants. The process was overseen by the Symbiosis Commission, which approved the applications of prospective hosts. Two of the Dax symbiont's previous hosts served on the Symbiosis Commission, and here's everything that has been revealed about them and the other official, and unofficial, Dax hosts including Jadzia in Star Trek: Deep Space Nine.
