A Finnish translator of movies was asked to take this vocabulary test.
http://testyourvocab.com/
I don't remember the exact result but it was between 16,000 and 17,000 words. I thought that was quite a low figure and took the test myself. I didn't fare much better, 22,400 words.
Maybe my higher expectations were the result of some comments I have heard from native speakers, including British and American lecturers at Helsinki University. I remember from my younger days some of them mentioning that they think they have a passive vocabulary of approximately 60,000 words. It says at the end of the above test that a native speaker's vocabulary is usually between 20,000 and 35,000 words.
As I live in Finland, I can't have continuous immersion in English even though I hear and see it daily. My interests have also shaped my vocabulary to a large extent. For example, I never read or talk about kitchen utensils and consequently a 5-year-old native speaker of English knows more such words than I. I just looked up the word
rolling pin as an example of a word I didn't know.

I am also unfamiliar with many colloquial words frequently used by native speakers.
On the other hand, I do know some words that few natives know. There are two reasons for this. I took Latin for three years in school and, as that wasn't enough for the professors of English Philology at Helsinki University, I took some additional Latin courses at university — and of course I had to pass Latin language tests as well. About 60 percent of English vocabulary derives from Latin, either direct or through French. Knowing some Latin helps one get into the core of many English words. A rare English word may be derived from a very common Latin word.
Another reason for my knowing some rare English words is the fact that when I was very young, I had pen pals in English-speaking countries. My vocabulary was very small in those early days and therefore I often had to consult a dictionary. Dictionaries don't necessarily mention that a word is rare, so often there was no way for me to know whether the word I was going to use was a common one or not.
One of the words I learned this way was
tintinnabulation. I think my pen friend had to look the word up when she read it in my letter.

If you take the test, be honest. Don't check words you
think you know. Check only those you
do know.
CB