Typically native speakers know 15,000 to 20,000 word families - or lemmas - in their first language.
Word family/lemma is a root word and all its inflections, for example: run, running, ran; blue, bluer, bluest, blueish, etc.
So does someone who can hold a decent conversation in a second language know 15,000 to 20,000 words? Is this a realistic goal for our listener to aim for? Unlikely.
Prof Webb found that people who have been studying languages in a traditional setting - say French in Britain or English in Japan - often struggle to learn more than 2,000 to 3,000 words, even after years of study.
In fact, a study in Taiwan showed that after nine years of learning a foreign language half of the students failed to learn the most frequently-used 1,000 words.