The original Tekken arcade version had eight playable fighters. The 1994 arcade release presented a compact roster, with each selectable fighter linked to a particular sub-boss encountered later in Arcade Mode. Heihachi Mishima served as the final opponent.
The eight-character lineup was expanded substantially for the PlayStation conversion, released in Japan in 1995. That home version made the sub-bosses and Heihachi unlockable, and added Devil as an unlockable form of Kazuya, bringing the playable total to 18.
This is the main source of confusion in the question: 18 is the correct count for the original PlayStation version, not for the original arcade game. The arcade edition also reflects Tekken's early design as a relatively streamlined 3D fighter, created to compete with Sega's Virtua Fighter.
Tekken was released for arcades on Namco System 11 hardware, which was closely related to PlayStation technology. That shared architecture helped Namco convert the game to the console quickly while adding characters, modes, arranged music, and ending movies.