Nine members originally set out from Rivendell to destroy the One Ring. Together, they were known as the Fellowship of the Ring, or the Company of the Ring.
The Fellowship consisted of Frodo Baggins, the Ring-bearer; Samwise Gamgee, Merry Brandybuck, and Pippin Took; Aragorn; Boromir; Legolas; Gimli; and Gandalf. This group represented Hobbits, Men, an Elf, a Dwarf, and a Wizard, bringing together several of Middle-earth's free peoples.
The number nine was deliberately chosen to oppose the nine Ringwraiths, or Black Riders, who hunted the Ring for Sauron. The Fellowship began its journey south from Rivendell, but it did not remain united for the entire quest. Gandalf fell in Moria, Boromir died defending Merry and Pippin, and the group eventually broke apart at Amon Hen.
The phrase “originally set out” matters: later events split the Fellowship into smaller groups, but the company that departed Rivendell contained nine walkers.