29,500, non-native English speaker (but studied in the US). Retook the test and omitted all the words I could not define with total confidence on the spot; the original score was 31,400. The test is peculiar in that the distribution appears to be uneven. Subjectively there is a sharp break between words that one would know from Shakespeare, Tolkien and Dunsany, and words no one would ever know unless they studied the OED. For statistical significance they would need more words.