According to the test, it is about 29k, though, like Rev said above. A lot of these words are rarely used, and I had to think about them, but I knew their definition even if I had never regularly (or ever) used them before.
Around 25k. There were some words I knew and could vaguely remember the context and assume the general meaning of it, but I didn't check them since I couldn't tell you off the top of my head.
Still, I think I did pretty good especially since English is not my first language.