What is the minimal size of vocabulary that could enable two persons to communicate more or less sensibly about matters of everyday life in English? Would 3000 words (8000 words, if different grammatical forms are taken into account) be a reasonable estimate?
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This link here gives some interesting figures on words needed for comprehension and coverage. It's mostly related to written text, but it does give a nice comparison of "regular" reading versus something like a teen novel, and about 1/3 of the way down the page it suggests a vocabulary of ~ 2,000-3,000.
It also gives breakdowns for newspapers, texts, novels, etc.