One Direction had five members when they formed.
Niall Horan, Zayn Malik, Liam Payne, Harry Styles and Louis Tomlinson were originally auditioning as solo contestants on the 2010 series of The X Factor. After they failed to advance individually in the Boys category, judges combined them into a five-member group for the Groups category.
The band was formed on 23 July 2010, and Styles is commonly credited with suggesting the name One Direction. Their first performance as a group was an acoustic version of “Torn” at the judges’ houses stage. Although they finished third in the competition, the group soon signed with Simon Cowell’s Syco Records.
The number changed in March 2015, when Zayn Malik left the band, making One Direction a four-piece. The remaining members continued without replacing him until they began an indefinite hiatus in 2016. Thus, “five” refers specifically to the lineup at formation, not the later post-Malik lineup.