https://www.quora.com/Can-fluency-in...-words-learned
It gives these rough figures:
1. 400 - 500 words are necessary for basic conversations.
2. 800 - 1000 words are necessary for basic reading.
3. 1,500 - 2,000 words are enough for all day activities and sufficient enough for average reading.
4. 3,000 - 4,000 - enough to read newspapers and specialized reading.
5. 8,000 words and more is enough for nearly fluent conversations, reading and listening.
6. 10,000 - 20,000 words - active vocabulary of a modern European.
7. 50,000 - 100,000 words - passive vocabulary of a modern European.