How much should I know?
There are approximately 170,000 words (even more if you include the obsoletes) in the English Language.
If you know between 1,000 to 3,000 words, you can have a conversation with other people. And If you know around 10,000 words, you're considered fluent - that's around 5.8% of the language domain.
Now apply that to other domains - physics, biology, and technology.
The volume can help you become fluent, but only to a certain point.
It's how you use what you've learned that matters most. Beginners can still be experts if they put what they learn into practice.