Eighteen is the usual number of Luohans, also known as Arhats, 'Destroyers of the passions', or 'Worthiness'. Luohans are Buddha's apostles, probably originated in Kashmir and were first mentioned as sixteen Arhats in the Mahayanavataraka, which was translated into Chinese in AD 437.
A full transcription of the sixteen names were given in AD 653 by the pilgrim Xuan Zang, leaving the last two to be explained. These additions were probably adopted by the end of the 10th Century, with the Arhats who tamed the Dragon and the Tiger. These two animals are symbolic of the East and West respectively. They could also be seen as the triumph of Buddhism over Daoism, as the dragon and tiger are Daoist symbols. The Eighteen Luohans have distinguishing symbols, badges or poses but there are considerable variations on the theme.