Adding the source for this. http://iteslj.org/Articles/Cervatiuc-VocabularyAcquisition.html
According to this, 17,000 word families. The statistic is slightly less than Randy's estimate but close enough.
February 3, 2013
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"Most Native English adult speakers who have taken the test fall in the range 20.000-35.000 words; and for foreign learners of English, the most common vocabulary size is from 2.500-9.000 words".
How many words do YOU know? Try this: http://testyourvocab.com
February 3, 2013
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A college-educated person in the United states knows 20,000 word families, on average. Source: "Learning Vocabulary in a Foreign Language", by I.S.P. Nation.
February 3, 2013
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COLDSTONE
Language Skills
Chinese (Mandarin), Chinese (Cantonese), Chinese (Other), English, French, Japanese, Spanish