Fluency is the goal for many people studying English, but it is hard to measure. Why would it be hard to measure? Because students have different end goals for their language skills. If you want to be a medical doctor, fluency standards are going to be higher with a lot of specialized vocabulary. If I want to be an English teacher, I'm going to need high levels of accuracy in all my uses of English. If I want to be an accountant, again, my goals will be different.
How many words are in English?
English has 600,000 word families. A word family describes a set of words that are all related. Let's look at the word information.
This chart describes the word family of "information"