HOW MANY words must one know in order to use English effectively? The answer depends on several variables, including the definition of "word."
For the purpose of counting, a word can be defined as the kind of lexical unit a person has to learn; all the derivative and compound forms that are merely morphological variations on the conceptual theme would not count as separate words. For example, write is a word and its morphological variants (writes, writ, wrote, written, writing, writer and so on) are relatives in the same family. If such a family is counted as a single word and knowing a word is defined as being able to recognize which of four definitions is closest to the meaning, the reading vocabulary of the average high school graduate should consist of about 40,000 words. If all the proper names and all the idiomatic expressions are also counted as words, that estimate would have to be doubled.
This figure says something about the ability of children to learn words. If the average high school graduate is 17 years old, the 80,000 words must have been learned over a period of 16 years. Hence the average child learns at the rate of 5,000 words per year, or about 13 per day. Children with large vocabularies probably pick up new words at twice that rate. Clearly a learning process of great complexity goes on at a rapid rate in every normal child.