There are several ways to try and answer this question, but most studies seem to indicate that if you know between 2,000 and 3,000 word in a given language you can function in most any environment. Some of the better studies conducted by teachers of ESL (English as a Second Language) show that foreign students who acquire a vocabulary of the 2,500 most commonly used words in English can handle any university-level academic text. The percentage of technical terms they encounter is less than 1% of the total vocabulary, and even their native English-speaking classmates will probably need to look up many of those words. Once your vocabulary reaches a certain size, you can pick up words by context, or if someone tells you the definition, you'll know enough to understand it. The goal is to acquire a big enough vocabulary that it can build itself.