Bluebulb Projects' The Measure of Things presents:
18,000 is about one-and-nine-tenths times the number of Written Characters in the Chinese Language.
In other words, it's 1.925 times the count of Written Characters in the Chinese Language, and the count of Written Characters in the Chinese Language is 0.5195 times that amount.
(zhōngwén, 中文) (Hànzì, 汉字/漢字; Shuowen Jiezi count; distinct characters)
According to the Han Dynasty dictionary, Shuowen Jiezi (說文解字), there are 9,353 distinct Han characters used to depict the Chinese language. Fluency in written Chinese requires mastery of 4,000 - 5,000 of these characters.