My students are sometimes surprised when I confess that I don't know all the words in the English language. English is a vast language with thousands and thousands of words so nobody knows all of them. Fortunately I have a whole range of dictionaries and online resources to help me discover and explain what words mean and how they are used.
It is important to remember that nobody actually needs to know what every word means. Part of learning a foreign language is developing strategies to deal with new words. Strategies such as guessing the meaning from context or simply ignoring the more unusual words. It's quite possible to understand a piece of writing without understanding every single word. Language students have to prioritise what they want to learn.
Back to the original question, how many words do you know? The University of Ghent are undertaking research which will help you to estimate this. I just did the test for the second or third time and it turns out that I know approximately 76% of all English words! I'm fairly sure it is probably less but am pleased that I was able to avoid the 'fictitious words' invented by the authors of the research.
It's a fun test to take and it helps with scientific research.
2023 Update - sadly the test is no longer available.