[monopede]

English is my second language and I got ~13,500 words. I was surprised that this was only half the average. I read a lot of scientific papers in my field and almost never come across a word I don't know. Writing technical papers well also requires not much vocabulary (it's more about clarity of expression). I believe to get beyond that number you simply have to read lots of books. I notice lots of words I've never heard of when reading, say, Bill Bryson but I very rarely bother to look them up because they usually make enough sense from the context.