How Many Words Do You Need to Speak a Foreign Language

In a forum I saw some people discussing how many words a person needs to talk with native speakers of the foreign language that they're learning. Some people argue that since the average person only uses a few thousand words most of the time, that is all you need. Other people argue that it helps to have many thousands more uncommon words at the ready, to fill in the holes. The topic reminds me of this Google meme I saw wandering around Facebook. This screen capture is meant to be funny, but at the same time ... I'd argue for the idea that a larger vocabulary can make all the difference in what it is we're able to talk about. 3000 words might not be enough to even express to a hotel manager that the room you've been assigned has bed bugs.