Excellent article on: "How Many Words Do You Need to Know in ????? language)?
I have a big problem with studies like this. Just because you can "understand" the words does not mean you can extract meaning from the language. While being able to parse the words and grammar are a huge first step, if you don't know the idiom it won't matter if you understood 100% of the individual words.
Different articles show different figures. It seems so easy when you hear that with 2000 or 3000 words you are able to understand many things and speak quite freely. However, I believe that even with 10 000 words one may have problems to express himself/herself. Your native language you learn for years in all of the areas (biology, geography, history, math etc.). While learning foreign languages it is hard to acquire those words and expressions that you do not use daily.